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[PATCHv4,REBASED,3/4] x86/64/kexec: Map original relocate_kernel() in init_transition_pgtable()

Message ID 20241016111458.846228-4-kirill.shutemov@linux.intel.com
State New
Headers show
Series x86: Reduce code duplication on page table initialization | expand

Commit Message

Kirill A. Shutemov Oct. 16, 2024, 11:14 a.m. UTC 
The init_transition_pgtable() function sets up transitional page tables.
It ensures that the relocate_kernel() function is present in the
identity mapping at the same location as in the kernel page tables.
relocate_kernel() switches to the identity mapping, and the function
must be present at the same location in the virtual address space before
and after switching page tables.

init_transition_pgtable() maps a copy of relocate_kernel() in
image->control_code_page at the relocate_kernel() virtual address, but
the original physical address of relocate_kernel() would also work.

It is safe to use original relocate_kernel() physical address cannot be
overwritten until swap_pages() is called, and the relocate_kernel()
virtual address will not be used by then.

Map the original relocate_kernel() at the relocate_kernel() virtual
address in the identity mapping. It is preparation to replace the
init_transition_pgtable() implementation with a call to
kernel_ident_mapping_init().

Note that while relocate_kernel() switches to the identity mapping, it
does not flush global TLB entries (CR4.PGE is not cleared). This means
that in most cases, the kernel still runs relocate_kernel() from the
original physical address before the change.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
---
 arch/x86/kernel/machine_kexec_64.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Borislav Petkov Oct. 31, 2024, 2:23 p.m. UTC | #1 
On Wed, Oct 16, 2024 at 02:14:57PM +0300, Kirill A. Shutemov wrote:
> The init_transition_pgtable() function sets up transitional page tables.
> It ensures that the relocate_kernel() function is present in the
> identity mapping at the same location as in the kernel page tables.
> relocate_kernel() switches to the identity mapping, and the function
> must be present at the same location in the virtual address space before
> and after switching page tables.
> 
> init_transition_pgtable() maps a copy of relocate_kernel() in
> image->control_code_page at the relocate_kernel() virtual address, but
> the original physical address of relocate_kernel() would also work.
> 
> It is safe to use original relocate_kernel() physical address cannot be
					^^^^^^^^^^^^^^^

something went missing here in that sentence. Reads weird.

> overwritten until swap_pages() is called, and the relocate_kernel()
> virtual address will not be used by then.

...

> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 9c9ac606893e..645690e81c2d 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -157,7 +157,7 @@ static int init_transition_pgtable(struct kimage *image, pgd_t *pgd)
>  	pte_t *pte;
>  
>  	vaddr = (unsigned long)relocate_kernel;
> -	paddr = __pa(page_address(image->control_code_page)+PAGE_SIZE);
> +	paddr = __pa(relocate_kernel);
>  	pgd += pgd_index(vaddr);
>  	if (!pgd_present(*pgd)) {
>  		p4d = (p4d_t *)get_zeroed_page(GFP_KERNEL);

Such changes always make me nervous so I'd queue them only after this merge
window is over so that they can get maximal testing in next. Unless someone
objects...
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Patch

diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index 9c9ac606893e..645690e81c2d 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -157,7 +157,7 @@  static int init_transition_pgtable(struct kimage *image, pgd_t *pgd)
 	pte_t *pte;
 
 	vaddr = (unsigned long)relocate_kernel;
-	paddr = __pa(page_address(image->control_code_page)+PAGE_SIZE);
+	paddr = __pa(relocate_kernel);
 	pgd += pgd_index(vaddr);
 	if (!pgd_present(*pgd)) {
 		p4d = (p4d_t *)get_zeroed_page(GFP_KERNEL);