[1/2] x86/numa: Introduce numa_fill_memblks()

From: Alison Schofield <alison.schofield@intel.com>

numa_fill_memblks() fills in the gaps in numa_meminfo memblks
over an HPA address range.

The initial use case is the ACPI driver that needs to extend
SRAT defined proximity domains to an entire CXL CFMWS Window[1].

The APCI driver expects to use numa_fill_memblks() while parsing
the CFMWS. Extending the memblks created during SRAT parsing, to
cover the entire CFMWS Window, is desirable because everything in
a CFMWS Window is expected to be of a similar performance class.

Requires CONFIG_NUMA_KEEP_MEMINFO.

[1] A CXL CFMWS Window represents a contiguous CXL memory resource,
aka an HPA range. The CFMWS (CXL Fixed Memory Window Structure) is
part of the ACPI CEDT (CXL Early Discovery Table).

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
---
 arch/x86/include/asm/sparsemem.h |  2 +
 arch/x86/mm/numa.c               | 82 ++++++++++++++++++++++++++++++++
 include/linux/numa.h             |  7 +++
 3 files changed, 91 insertions(+)

On 5/18/23 17:04, alison.schofield@intel.com wrote:
> The initial use case is the ACPI driver that needs to extend
> SRAT defined proximity domains to an entire CXL CFMWS Window[1].

Dumb question time: Why didn't the SRAT just cover this sucker in the
first place?  Are we fixing up a BIOS bug or is there a legitimate
reason that the SRAT didn't cover it up front?

On 5/18/23 17:26, Alison Schofield wrote:
> On Thu, May 18, 2023 at 05:08:16PM -0700, Dave Hansen wrote:
>> On 5/18/23 17:04, alison.schofield@intel.com wrote:
>>> The initial use case is the ACPI driver that needs to extend
>>> SRAT defined proximity domains to an entire CXL CFMWS Window[1].
>>
>> Dumb question time: Why didn't the SRAT just cover this sucker in the
>> first place?  Are we fixing up a BIOS bug or is there a legitimate
>> reason that the SRAT didn't cover it up front?
>>
> There is no requirement that the BIOS describe (in the SRAT) all the
> HPA assigned to a CFMWS Window. The HPA range may not actually map to
> any memory at boot time. It can be persistent capacity or may be there
> to enable hot-plug. IIUC BIOS can pick and choose and define volatile
> regions wherever it pleases.

I understand that it _can_ do this.  I'm trying to get to the reasoning
of why.

Is this essentially so that the physical address space doesn't have to
be *committed* to a single use up front?  For RAM, I guess this wasn't a
problem because there was only a finite amount of RAM that could get
hotplugged into a single node.

But with these fancy schmancy new devices, it's really hard to figure
out how much space will show up and what performance it will have until
you actually start poking at it.  The firmware wasn't _quite_ sure how
it wanted to burn the physical address space at the time the SRAT was
created.  But, now it knows, and this is handling the case where the
firmware only expands an adjacent chunk of physical address space.

Close?

alison.schofield@ wrote:
> From: Alison Schofield <alison.schofield@intel.com>
> 
> numa_fill_memblks() fills in the gaps in numa_meminfo memblks
> over an HPA address range.
> 
> The initial use case is the ACPI driver that needs to extend
> SRAT defined proximity domains to an entire CXL CFMWS Window[1].

I feel like this demands more explanation because the "need" is not
apparent. In fact its a Linux policy choice not a requirement. The next
patch has some of this, but this story is needed earlier for someone
that reads this patch first. Something like:

---

The CFWMS is an ACPI data structure that indicates *potential* locations
where CXL memory can be placed. It is the playground where the CXL
driver has free reign to establish regions.  That space can be populated
by BIOS created regions, or driver created regions, after hotplug or
other reconfiguration.

When the BIOS creates a region in a CXL Window it additionally describes
that subset of the Window range in the other typical ACPI tables SRAT,
SLIT, and HMAT. The rationale for the BIOS not pre-describing the entire
CXL Window in SRAT, SLIT, and HMAT is that it can not predict the
future. I.e. there is nothing stopping higher or lower performance
devices being placed in the same Window. Compare that to ACPI memory
hotplug that just onlines additional capacity in the proximity domain
with little freedom for dynamic performance differentiation.

That leaves the OS with a choice, should unpopulated window capacity
match the proximity domain of an existing region, or should it allocate
a new one? This patch takes the simple position of minimizing proximity
domain proliferation and reuse any proximity domain intersection for the
entire Window. If the Window has no intersections then allocate a new
proximity domain. Note that SRAT, SLIT and HMAT information can be
enumerated dynamically in a standard way from device provided data.
Think of CXL as the end of ACPI needing to describe memory attributes,
CXL offers a standard discovery model for performance attributes, but
Linux still needs to interoperate with the old regime.

---

> 
> The APCI driver expects to use numa_fill_memblks() while parsing

s/APCI/ACPI/

Again, the ACPI code does not have any expectation, this is pure OS
policy decision about how to handle undescribed memory.

> the CFMWS. Extending the memblks created during SRAT parsing, to
> cover the entire CFMWS Window, is desirable because everything in
> a CFMWS Window is expected to be of a similar performance class.
> 
> Requires CONFIG_NUMA_KEEP_MEMINFO.

Not sure this adds anything to the description.

> 
> [1] A CXL CFMWS Window represents a contiguous CXL memory resource,
> aka an HPA range. The CFMWS (CXL Fixed Memory Window Structure) is
> part of the ACPI CEDT (CXL Early Discovery Table).
> 
> Signed-off-by: Alison Schofield <alison.schofield@intel.com>
> ---
>  arch/x86/include/asm/sparsemem.h |  2 +
>  arch/x86/mm/numa.c               | 82 ++++++++++++++++++++++++++++++++
>  include/linux/numa.h             |  7 +++
>  3 files changed, 91 insertions(+)
> 
> diff --git a/arch/x86/include/asm/sparsemem.h b/arch/x86/include/asm/sparsemem.h
> index 64df897c0ee3..1be13b2dfe8b 100644
> --- a/arch/x86/include/asm/sparsemem.h
> +++ b/arch/x86/include/asm/sparsemem.h
> @@ -37,6 +37,8 @@ extern int phys_to_target_node(phys_addr_t start);
>  #define phys_to_target_node phys_to_target_node
>  extern int memory_add_physaddr_to_nid(u64 start);
>  #define memory_add_physaddr_to_nid memory_add_physaddr_to_nid
> +extern int numa_fill_memblks(u64 start, u64 end);
> +#define numa_fill_memblks numa_fill_memblks

What is this for? The other defines are due to being an arch-specific
API and the #define is how the arch declares that it has a local version
to replace the generic one.

>  #endif
>  #endif /* __ASSEMBLY__ */
>  
> diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
> index 2aadb2019b4f..6c8f9cff71da 100644
> --- a/arch/x86/mm/numa.c
> +++ b/arch/x86/mm/numa.c
> @@ -11,6 +11,7 @@
>  #include <linux/nodemask.h>
>  #include <linux/sched.h>
>  #include <linux/topology.h>
> +#include <linux/sort.h>
>  
>  #include <asm/e820/api.h>
>  #include <asm/proto.h>
> @@ -961,4 +962,85 @@ int memory_add_physaddr_to_nid(u64 start)
>  	return nid;
>  }
>  EXPORT_SYMBOL_GPL(memory_add_physaddr_to_nid);
> +
> +static int __init cmp_memblk(const void *a, const void *b)
> +{
> +	const struct numa_memblk *ma = *(const struct numa_memblk **)a;
> +	const struct numa_memblk *mb = *(const struct numa_memblk **)b;
> +
> +	if (ma->start != mb->start)
> +		return (ma->start < mb->start) ? -1 : 1;
> +
> +	if (ma->end != mb->end)
> +		return (ma->end < mb->end) ? -1 : 1;

Why is this sorting by start and end? I can maybe guess, but a comment
would help a future intrepid reader.

> +
> +	return 0;
> +}
> +
> +static struct numa_memblk *numa_memblk_list[NR_NODE_MEMBLKS] __initdata;
> +
> +/**
> + * numa_fill_memblks - Fill gaps in numa_meminfo memblks
> + * @start: address to begin fill
> + * @end: address to end fill
> + *
> + * Find and extend numa_meminfo memblks to cover the @start/@end
> + * HPA address range, following these rules:
> + * 1. The first memblk must start at @start
> + * 2. The last memblk must end at @end

Why these requirements? I worry this is too strict because of the
existence of numa_cleanup_meminfo() which indicates that Linux has seen
quite messy firmware tables, or otherwise needs to cleanup after the
"numa=fake=" command line option. Is it not enough to just check for any
intersection?

> + * 3. Fill the gaps between memblks by extending numa_memblk.end
> + * Result: All addresses in start/end range are included in
> + *	   numa_meminfo.
> + *
> + * RETURNS:
> + * 0		  : Success. numa_meminfo fully describes start/end
> + * NUMA_NO_MEMBLK : No memblk exists in start/end range

This probably wants to clarify whether @end is inclusive or exclusive.

> + */
> +
> +int __init numa_fill_memblks(u64 start, u64 end)
> +{
> +	struct numa_meminfo *mi = &numa_meminfo;
> +	struct numa_memblk **blk = &numa_memblk_list[0];
> +	int count = 0;
> +
> +	for (int i = 0; i < mi->nr_blks; i++) {
> +		struct numa_memblk *bi = &mi->blk[i];
> +
> +		if (start <= bi->start && end >= bi->end) {

Maybe a comment about what this is doing? This is looking for to see if
any CXL window completely overlaps any SRAT entry?
   
> +			blk[count] = &mi->blk[i];
> +			count++;
> +		}
> +	}
> +	if (!count)
> +		return NUMA_NO_MEMBLK;
> +
> +	if (count == 1) {
> +		blk[0]->start = start;
> +		blk[0]->end = end;
> +		return 0;

So this is updating numa_meminfo in place?

> +	}
> +
> +	sort(&blk[0], count, sizeof(blk[0]), cmp_memblk, NULL);
> +	blk[0]->start = start;
> +	blk[count - 1]->end = end;
> +
> +	for (int i = 0, j = 1; j < count; i++, j++) {
> +		/* Overlaps OK. sort() put the lesser end first */
> +		if (blk[i]->start == blk[j]->start)
> +			continue;
> +
> +		/* No gap */
> +		if (blk[i]->end == blk[j]->start)
> +			continue;
> +
> +		/* Fill the gap */
> +		if (blk[i]->end < blk[j]->start) {
> +			blk[i]->end = blk[j]->start;
> +			continue;
> +		}

This looks clever to sort an array of pointers into the existing
numa_meminfo, I think it needs some comments to explain the cleverness,
but I am not seeing anything glaringly wrong about the approach.

> +	}
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(numa_fill_memblks);
> +
>  #endif
> diff --git a/include/linux/numa.h b/include/linux/numa.h
> index 59df211d051f..0f512c0aba54 100644
> --- a/include/linux/numa.h
> +++ b/include/linux/numa.h
> @@ -12,6 +12,7 @@
>  #define MAX_NUMNODES    (1 << NODES_SHIFT)
>  
>  #define	NUMA_NO_NODE	(-1)
> +#define	NUMA_NO_MEMBLK	(-1)
>  
>  /* optionally keep NUMA memory info available post init */
>  #ifdef CONFIG_NUMA_KEEP_MEMINFO
> @@ -43,6 +44,12 @@ static inline int phys_to_target_node(u64 start)
>  	return 0;
>  }
>  #endif
> +#ifndef numa_fill_memblks
> +static inline int __init numa_fill_memblks(u64 start, u64 end)
> +{
> +	return NUMA_NO_MEMBLK;
> +}
> +#endif

Why does linux/numa.h need to care about this x86-specific init routine?

>  #else /* !CONFIG_NUMA */
>  static inline int numa_map_to_online_node(int node)
>  {
> -- 
> 2.37.3
>

On Sat, Jun 03, 2023 at 04:53:13PM -0700, Dan Williams wrote:
> alison.schofield@ wrote:
> > From: Alison Schofield <alison.schofield@intel.com>
> > 
> > numa_fill_memblks() fills in the gaps in numa_meminfo memblks
> > over an HPA address range.
> > 
> > The initial use case is the ACPI driver that needs to extend
> > SRAT defined proximity domains to an entire CXL CFMWS Window[1].
> 
> I feel like this demands more explanation because the "need" is not
> apparent. In fact its a Linux policy choice not a requirement. The next
> patch has some of this, but this story is needed earlier for someone
> that reads this patch first. Something like:
> 

Hi Dan,

Thanks for the review :)

Sure, I can add the story below to make the 'need' for this function
more apparent, as well as s/needs/want so as not to conflate need with
requirement.

> ---
> 
> The CFWMS is an ACPI data structure that indicates *potential* locations
> where CXL memory can be placed. It is the playground where the CXL
> driver has free reign to establish regions.  That space can be populated
> by BIOS created regions, or driver created regions, after hotplug or
> other reconfiguration.
> 
> When the BIOS creates a region in a CXL Window it additionally describes
> that subset of the Window range in the other typical ACPI tables SRAT,
> SLIT, and HMAT. The rationale for the BIOS not pre-describing the entire
> CXL Window in SRAT, SLIT, and HMAT is that it can not predict the
> future. I.e. there is nothing stopping higher or lower performance
> devices being placed in the same Window. Compare that to ACPI memory
> hotplug that just onlines additional capacity in the proximity domain
> with little freedom for dynamic performance differentiation.
> 
> That leaves the OS with a choice, should unpopulated window capacity
> match the proximity domain of an existing region, or should it allocate
> a new one? This patch takes the simple position of minimizing proximity
> domain proliferation and reuse any proximity domain intersection for the
> entire Window. If the Window has no intersections then allocate a new
> proximity domain. Note that SRAT, SLIT and HMAT information can be
> enumerated dynamically in a standard way from device provided data.
> Think of CXL as the end of ACPI needing to describe memory attributes,
> CXL offers a standard discovery model for performance attributes, but
> Linux still needs to interoperate with the old regime.
> 
> ---
> 
> > 
> > The APCI driver expects to use numa_fill_memblks() while parsing
> 
> s/APCI/ACPI/
> 
> Again, the ACPI code does not have any expectation, this is pure OS
> policy decision about how to handle undescribed memory.
> 

The intent was to show the pending use case, perhaps 'wants to' use
this function to enact a purely OS policy decision!


> > the CFMWS. Extending the memblks created during SRAT parsing, to
> > cover the entire CFMWS Window, is desirable because everything in
> > a CFMWS Window is expected to be of a similar performance class.
> > 
> > Requires CONFIG_NUMA_KEEP_MEMINFO.
> 
> Not sure this adds anything to the description.
> 
> > 
> > [1] A CXL CFMWS Window represents a contiguous CXL memory resource,
> > aka an HPA range. The CFMWS (CXL Fixed Memory Window Structure) is
> > part of the ACPI CEDT (CXL Early Discovery Table).
> > 
> > Signed-off-by: Alison Schofield <alison.schofield@intel.com>
> > ---
> >  arch/x86/include/asm/sparsemem.h |  2 +
> >  arch/x86/mm/numa.c               | 82 ++++++++++++++++++++++++++++++++
> >  include/linux/numa.h             |  7 +++
> >  3 files changed, 91 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/sparsemem.h b/arch/x86/include/asm/sparsemem.h
> > index 64df897c0ee3..1be13b2dfe8b 100644
> > --- a/arch/x86/include/asm/sparsemem.h
> > +++ b/arch/x86/include/asm/sparsemem.h
> > @@ -37,6 +37,8 @@ extern int phys_to_target_node(phys_addr_t start);
> >  #define phys_to_target_node phys_to_target_node
> >  extern int memory_add_physaddr_to_nid(u64 start);
> >  #define memory_add_physaddr_to_nid memory_add_physaddr_to_nid
> > +extern int numa_fill_memblks(u64 start, u64 end);
> > +#define numa_fill_memblks numa_fill_memblks
> 
> What is this for? The other defines are due to being an arch-specific
> API and the #define is how the arch declares that it has a local version
> to replace the generic one.

That define, along with the numa.h change below, are to support builds of
CONFIG_ARM64 and CONFIG_LOONGARCH, both include the caller acpi_parse_cfmws(),
of numa_fill_memblks().

> 
> >  #endif
> >  #endif /* __ASSEMBLY__ */
> >  
> > diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
> > index 2aadb2019b4f..6c8f9cff71da 100644
> > --- a/arch/x86/mm/numa.c
> > +++ b/arch/x86/mm/numa.c
> > @@ -11,6 +11,7 @@
> >  #include <linux/nodemask.h>
> >  #include <linux/sched.h>
> >  #include <linux/topology.h>
> > +#include <linux/sort.h>
> >  
> >  #include <asm/e820/api.h>
> >  #include <asm/proto.h>
> > @@ -961,4 +962,85 @@ int memory_add_physaddr_to_nid(u64 start)
> >  	return nid;
> >  }
> >  EXPORT_SYMBOL_GPL(memory_add_physaddr_to_nid);
> > +
> > +static int __init cmp_memblk(const void *a, const void *b)
> > +{
> > +	const struct numa_memblk *ma = *(const struct numa_memblk **)a;
> > +	const struct numa_memblk *mb = *(const struct numa_memblk **)b;
> > +
> > +	if (ma->start != mb->start)
> > +		return (ma->start < mb->start) ? -1 : 1;
> > +
> > +	if (ma->end != mb->end)
> > +		return (ma->end < mb->end) ? -1 : 1;
> 
> Why is this sorting by start and end? I can maybe guess, but a comment
> would help a future intrepid reader.

Sure, can add comment. It compares ends only if starts are the same.
It's putting the list in order for numa_fill_memblks() to walk and
fill.

> 
> > +
> > +	return 0;
> > +}
> > +
> > +static struct numa_memblk *numa_memblk_list[NR_NODE_MEMBLKS] __initdata;
> > +
> > +/**
> > + * numa_fill_memblks - Fill gaps in numa_meminfo memblks
> > + * @start: address to begin fill
> > + * @end: address to end fill
> > + *
> > + * Find and extend numa_meminfo memblks to cover the @start/@end
> > + * HPA address range, following these rules:
> > + * 1. The first memblk must start at @start
> > + * 2. The last memblk must end at @end
> 
> Why these requirements? I worry this is too strict because of the
> existence of numa_cleanup_meminfo() which indicates that Linux has seen
> quite messy firmware tables, or otherwise needs to cleanup after the
> "numa=fake=" command line option. Is it not enough to just check for any
> intersection?

Yes, it would be enough to just check for intersection, and not
force the alignment. Will change code to reflect.

> 
> > + * 3. Fill the gaps between memblks by extending numa_memblk.end
> > + * Result: All addresses in start/end range are included in
> > + *	   numa_meminfo.
> > + *
> > + * RETURNS:
> > + * 0		  : Success. numa_meminfo fully describes start/end
> > + * NUMA_NO_MEMBLK : No memblk exists in start/end range
> 
> This probably wants to clarify whether @end is inclusive or exclusive.

It's exclusive and I'll add comment.

> 
> > + */
> > +
> > +int __init numa_fill_memblks(u64 start, u64 end)
> > +{
> > +	struct numa_meminfo *mi = &numa_meminfo;
> > +	struct numa_memblk **blk = &numa_memblk_list[0];
> > +	int count = 0;
> > +
> > +	for (int i = 0; i < mi->nr_blks; i++) {
> > +		struct numa_memblk *bi = &mi->blk[i];
> > +
> > +		if (start <= bi->start && end >= bi->end) {
> 
> Maybe a comment about what this is doing? This is looking for to see if
> any CXL window completely overlaps any SRAT entry?

Based on your first comment about messy tables, I can see the need to
expand the search to include any intersection. Will do.

>    
> > +			blk[count] = &mi->blk[i];
> > +			count++;
> > +		}
> > +	}
> > +	if (!count)
> > +		return NUMA_NO_MEMBLK;
> > +
> > +	if (count == 1) {
> > +		blk[0]->start = start;
> > +		blk[0]->end = end;
> > +		return 0;
> 
> So this is updating numa_meminfo in place?

Yes.

> 
> > +	}
> > +
> > +	sort(&blk[0], count, sizeof(blk[0]), cmp_memblk, NULL);
> > +	blk[0]->start = start;
> > +	blk[count - 1]->end = end;
> > +
> > +	for (int i = 0, j = 1; j < count; i++, j++) {
> > +		/* Overlaps OK. sort() put the lesser end first */
> > +		if (blk[i]->start == blk[j]->start)
> > +			continue;
> > +
> > +		/* No gap */
> > +		if (blk[i]->end == blk[j]->start)
> > +			continue;
> > +
> > +		/* Fill the gap */
> > +		if (blk[i]->end < blk[j]->start) {
> > +			blk[i]->end = blk[j]->start;
> > +			continue;
> > +		}
> 
> This looks clever to sort an array of pointers into the existing
> numa_meminfo, I think it needs some comments to explain the cleverness,
> but I am not seeing anything glaringly wrong about the approach.
> 

I'll add comments on all the above.


> > +	}
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(numa_fill_memblks);
> > +
> >  #endif
> > diff --git a/include/linux/numa.h b/include/linux/numa.h
> > index 59df211d051f..0f512c0aba54 100644
> > --- a/include/linux/numa.h
> > +++ b/include/linux/numa.h
> > @@ -12,6 +12,7 @@
> >  #define MAX_NUMNODES    (1 << NODES_SHIFT)
> >  
> >  #define	NUMA_NO_NODE	(-1)
> > +#define	NUMA_NO_MEMBLK	(-1)
> >  
> >  /* optionally keep NUMA memory info available post init */
> >  #ifdef CONFIG_NUMA_KEEP_MEMINFO
> > @@ -43,6 +44,12 @@ static inline int phys_to_target_node(u64 start)
> >  	return 0;
> >  }
> >  #endif
> > +#ifndef numa_fill_memblks
> > +static inline int __init numa_fill_memblks(u64 start, u64 end)
> > +{
> > +	return NUMA_NO_MEMBLK;
> > +}
> > +#endif
> 
> Why does linux/numa.h need to care about this x86-specific init routine?
> 

This is how I got ARM64 and LOONGARCH to build.


> >  #else /* !CONFIG_NUMA */
> >  static inline int numa_map_to_online_node(int node)
> >  {
> > -- 
> > 2.37.3
> > 
> 
>

[ add Mike, see "[Mike]" note below... ]

Alison Schofield wrote:
> On Sat, Jun 03, 2023 at 04:53:13PM -0700, Dan Williams wrote:
> > alison.schofield@ wrote:
> > > From: Alison Schofield <alison.schofield@intel.com>
> > > 
> > > numa_fill_memblks() fills in the gaps in numa_meminfo memblks
> > > over an HPA address range.
> > > 
> > > The initial use case is the ACPI driver that needs to extend
> > > SRAT defined proximity domains to an entire CXL CFMWS Window[1].
> > 
> > I feel like this demands more explanation because the "need" is not
> > apparent. In fact its a Linux policy choice not a requirement. The next
> > patch has some of this, but this story is needed earlier for someone
> > that reads this patch first. Something like:
> > 
> 
> Hi Dan,
> 
> Thanks for the review :)
> 
> Sure, I can add the story below to make the 'need' for this function
> more apparent, as well as s/needs/want so as not to conflate need with
> requirement.
> 
> > ---
> > 
> > The CFWMS is an ACPI data structure that indicates *potential* locations
> > where CXL memory can be placed. It is the playground where the CXL
> > driver has free reign to establish regions.  That space can be populated
> > by BIOS created regions, or driver created regions, after hotplug or
> > other reconfiguration.
> > 
> > When the BIOS creates a region in a CXL Window it additionally describes
> > that subset of the Window range in the other typical ACPI tables SRAT,
> > SLIT, and HMAT. The rationale for the BIOS not pre-describing the entire
> > CXL Window in SRAT, SLIT, and HMAT is that it can not predict the
> > future. I.e. there is nothing stopping higher or lower performance
> > devices being placed in the same Window. Compare that to ACPI memory
> > hotplug that just onlines additional capacity in the proximity domain
> > with little freedom for dynamic performance differentiation.
> > 
> > That leaves the OS with a choice, should unpopulated window capacity
> > match the proximity domain of an existing region, or should it allocate
> > a new one? This patch takes the simple position of minimizing proximity
> > domain proliferation and reuse any proximity domain intersection for the
> > entire Window. If the Window has no intersections then allocate a new
> > proximity domain. Note that SRAT, SLIT and HMAT information can be
> > enumerated dynamically in a standard way from device provided data.
> > Think of CXL as the end of ACPI needing to describe memory attributes,
> > CXL offers a standard discovery model for performance attributes, but
> > Linux still needs to interoperate with the old regime.
> > 
> > ---
> > 
> > > 
> > > The APCI driver expects to use numa_fill_memblks() while parsing
> > 
> > s/APCI/ACPI/
> > 
> > Again, the ACPI code does not have any expectation, this is pure OS
> > policy decision about how to handle undescribed memory.
> > 
> 
> The intent was to show the pending use case, perhaps 'wants to' use
> this function to enact a purely OS policy decision!

Sounds good, yeah I tend to read "need" as a requirement and assume that
Linux is out of spec or something breaks if it does not do the needed
thing.

> 
> 
> > > the CFMWS. Extending the memblks created during SRAT parsing, to
> > > cover the entire CFMWS Window, is desirable because everything in
> > > a CFMWS Window is expected to be of a similar performance class.
> > > 
> > > Requires CONFIG_NUMA_KEEP_MEMINFO.
> > 
> > Not sure this adds anything to the description.
> > 
> > > 
> > > [1] A CXL CFMWS Window represents a contiguous CXL memory resource,
> > > aka an HPA range. The CFMWS (CXL Fixed Memory Window Structure) is
> > > part of the ACPI CEDT (CXL Early Discovery Table).
> > > 
> > > Signed-off-by: Alison Schofield <alison.schofield@intel.com>
> > > ---
> > >  arch/x86/include/asm/sparsemem.h |  2 +
> > >  arch/x86/mm/numa.c               | 82 ++++++++++++++++++++++++++++++++
> > >  include/linux/numa.h             |  7 +++
> > >  3 files changed, 91 insertions(+)
> > > 
> > > diff --git a/arch/x86/include/asm/sparsemem.h b/arch/x86/include/asm/sparsemem.h
> > > index 64df897c0ee3..1be13b2dfe8b 100644
> > > --- a/arch/x86/include/asm/sparsemem.h
> > > +++ b/arch/x86/include/asm/sparsemem.h
> > > @@ -37,6 +37,8 @@ extern int phys_to_target_node(phys_addr_t start);
> > >  #define phys_to_target_node phys_to_target_node
> > >  extern int memory_add_physaddr_to_nid(u64 start);
> > >  #define memory_add_physaddr_to_nid memory_add_physaddr_to_nid
> > > +extern int numa_fill_memblks(u64 start, u64 end);
> > > +#define numa_fill_memblks numa_fill_memblks
> > 
> > What is this for? The other defines are due to being an arch-specific
> > API and the #define is how the arch declares that it has a local version
> > to replace the generic one.
> 
> That define, along with the numa.h change below, are to support builds of
> CONFIG_ARM64 and CONFIG_LOONGARCH, both include the caller acpi_parse_cfmws(),
> of numa_fill_memblks().

[Mike]

Hmm, ok, but this is piling onto the maintenance burden of x86 not
getting onboard with memblock for numa info yet. At a minimum that
avoidance of touching the ARM64 and LOONGARCH cases needs to be called
out, but it would be useful to have a discussion about the options here
with questions like:

- What's blocking x86 from switching to memblock?

- Or, does the memblock API support what numa_fill_memblks() wants to
  do? I.e. add a real numa_fill_memblks() implementation to
  drivers/base/arch_numa.c rather than skip SRAT based fixups for the
  generic case.

Last I remember it was the conceptual disconnect of x86 not marking Reserved
ranges as memory like other architectures:

https://lore.kernel.org/all/20200708091520.GE128651@kernel.org/

...but its been a while since this last came up and I have not been
following memblock developments. Maybe the anwser is the same in the
end, add x86-specific numa_fill_memblks, but this is as good a time as
any to revisit carrying that burden.

[..]
snipped the rest, looks like we are aligned there.

On Tue, Jun 06, 2023 at 01:45:35PM -0700, Dan Williams wrote:
> [ add Mike, see "[Mike]" note below... ]
> 
> Alison Schofield wrote:
> > On Sat, Jun 03, 2023 at 04:53:13PM -0700, Dan Williams wrote:
> > > alison.schofield@ wrote:
> > > > From: Alison Schofield <alison.schofield@intel.com>
> > > > 
> > > > numa_fill_memblks() fills in the gaps in numa_meminfo memblks
> > > > over an HPA address range.
> > > > 
> > > > The initial use case is the ACPI driver that needs to extend
> > > > SRAT defined proximity domains to an entire CXL CFMWS Window[1].
> > > 
> > > I feel like this demands more explanation because the "need" is not
> > > apparent. In fact its a Linux policy choice not a requirement. The next
> > > patch has some of this, but this story is needed earlier for someone
> > > that reads this patch first. Something like:
> > > 
> > 
> > Hi Dan,
> > 
> > Thanks for the review :)
> > 
> > Sure, I can add the story below to make the 'need' for this function
> > more apparent, as well as s/needs/want so as not to conflate need with
> > requirement.
> > 
> > > ---
> > > 
> > > The CFWMS is an ACPI data structure that indicates *potential* locations
> > > where CXL memory can be placed. It is the playground where the CXL
> > > driver has free reign to establish regions.  That space can be populated
> > > by BIOS created regions, or driver created regions, after hotplug or
> > > other reconfiguration.
> > > 
> > > When the BIOS creates a region in a CXL Window it additionally describes
> > > that subset of the Window range in the other typical ACPI tables SRAT,
> > > SLIT, and HMAT. The rationale for the BIOS not pre-describing the entire
> > > CXL Window in SRAT, SLIT, and HMAT is that it can not predict the
> > > future. I.e. there is nothing stopping higher or lower performance
> > > devices being placed in the same Window. Compare that to ACPI memory
> > > hotplug that just onlines additional capacity in the proximity domain
> > > with little freedom for dynamic performance differentiation.
> > > 
> > > That leaves the OS with a choice, should unpopulated window capacity
> > > match the proximity domain of an existing region, or should it allocate
> > > a new one? This patch takes the simple position of minimizing proximity
> > > domain proliferation and reuse any proximity domain intersection for the
> > > entire Window. If the Window has no intersections then allocate a new
> > > proximity domain. Note that SRAT, SLIT and HMAT information can be
> > > enumerated dynamically in a standard way from device provided data.
> > > Think of CXL as the end of ACPI needing to describe memory attributes,
> > > CXL offers a standard discovery model for performance attributes, but
> > > Linux still needs to interoperate with the old regime.
> > > 
> > > ---
> > > 
> > > > 
> > > > The APCI driver expects to use numa_fill_memblks() while parsing
> > > 
> > > s/APCI/ACPI/
> > > 
> > > Again, the ACPI code does not have any expectation, this is pure OS
> > > policy decision about how to handle undescribed memory.
> > > 
> > 
> > The intent was to show the pending use case, perhaps 'wants to' use
> > this function to enact a purely OS policy decision!
> 
> Sounds good, yeah I tend to read "need" as a requirement and assume that
> Linux is out of spec or something breaks if it does not do the needed
> thing.
> 
> > 
> > 
> > > > the CFMWS. Extending the memblks created during SRAT parsing, to
> > > > cover the entire CFMWS Window, is desirable because everything in
> > > > a CFMWS Window is expected to be of a similar performance class.
> > > > 
> > > > Requires CONFIG_NUMA_KEEP_MEMINFO.
> > > 
> > > Not sure this adds anything to the description.
> > > 
> > > > 
> > > > [1] A CXL CFMWS Window represents a contiguous CXL memory resource,
> > > > aka an HPA range. The CFMWS (CXL Fixed Memory Window Structure) is
> > > > part of the ACPI CEDT (CXL Early Discovery Table).
> > > > 
> > > > Signed-off-by: Alison Schofield <alison.schofield@intel.com>
> > > > ---
> > > >  arch/x86/include/asm/sparsemem.h |  2 +
> > > >  arch/x86/mm/numa.c               | 82 ++++++++++++++++++++++++++++++++
> > > >  include/linux/numa.h             |  7 +++
> > > >  3 files changed, 91 insertions(+)
> > > > 
> > > > diff --git a/arch/x86/include/asm/sparsemem.h b/arch/x86/include/asm/sparsemem.h
> > > > index 64df897c0ee3..1be13b2dfe8b 100644
> > > > --- a/arch/x86/include/asm/sparsemem.h
> > > > +++ b/arch/x86/include/asm/sparsemem.h
> > > > @@ -37,6 +37,8 @@ extern int phys_to_target_node(phys_addr_t start);
> > > >  #define phys_to_target_node phys_to_target_node
> > > >  extern int memory_add_physaddr_to_nid(u64 start);
> > > >  #define memory_add_physaddr_to_nid memory_add_physaddr_to_nid
> > > > +extern int numa_fill_memblks(u64 start, u64 end);
> > > > +#define numa_fill_memblks numa_fill_memblks
> > > 
> > > What is this for? The other defines are due to being an arch-specific
> > > API and the #define is how the arch declares that it has a local version
> > > to replace the generic one.
> > 
> > That define, along with the numa.h change below, are to support builds of
> > CONFIG_ARM64 and CONFIG_LOONGARCH, both include the caller acpi_parse_cfmws(),
> > of numa_fill_memblks().
> 
> [Mike]
> 
> Hmm, ok, but this is piling onto the maintenance burden of x86 not
> getting onboard with memblock for numa info yet. At a minimum that
> avoidance of touching the ARM64 and LOONGARCH cases needs to be called
> out, but it would be useful to have a discussion about the options here
> with questions like:
> 
> - What's blocking x86 from switching to memblock?

To start with, someone need to work on it :)

There are some differences in how drivers/base/arch_numa.c and
arch/x86/mm/numa.c handle SRAT ranges. E.g. x86 checks that SRAT covers all
the memory reported by e820 and have this peculiar dance around hotplugable
memory for the sake of movable_node.
Another x86 specific thing that is build around numa_meminfo is the
numa_emulation.

I don't see a conceptual reason why arch_numa.c cannot handle x86, but
that's quite some work needed to make that happen.
 
> - Or, does the memblock API support what numa_fill_memblks() wants to
>   do? I.e. add a real numa_fill_memblks() implementation to
>   drivers/base/arch_numa.c rather than skip SRAT based fixups for the
>   generic case.
 
memblock does not have a notion of empty physical ranges, so it will
require a new set of regions to support what numa_fill_memblks() wants to
do.

With this patch numa_meminfo essentially becomes a superset of
memblock.memory and to have a generic implementation in
drivers/base/arch_numa.c this set should be kept somewhere.

> Last I remember it was the conceptual disconnect of x86 not marking Reserved
> ranges as memory like other architectures:
> 
> https://lore.kernel.org/all/20200708091520.GE128651@kernel.org/

This was more about e820 vs memblock, I don't think it's relevant here.
 
> ...but its been a while since this last came up and I have not been
> following memblock developments. Maybe the anwser is the same in the
> end, add x86-specific numa_fill_memblks, but this is as good a time as
> any to revisit carrying that burden.

I've been thinking about how to make arch_numa to support x86 and (sigh)
loongarch, and the simplest way looks like shoving numa_meminfo there and
then optimizing redundant pieces.

For CXL on arm64/riscv we'd need a new data structure for empty physical
ranges anyway at some point and numa_meminfo quite fits the requirements.
We can later reconsider numa_meminfo vs memblock relationship.

That said, add x86-specific numa_fill_memblks and revisit this later is a
option as well :)