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[Xen-devel,v3,21/28] xen/arm32: head: Rework and document setup_fixmap()

Message ID 20190812173019.11956-22-julien.grall@arm.com
State Superseded
Headers show
Series xen/arm: Rework head.S to make it more compliant with the Arm Arm | expand

Commit Message

Julien Grall Aug. 12, 2019, 5:30 p.m. UTC 
At the moment, the fixmap table is only hooked when earlyprintk is used.
This is fine today because in C land, the fixmap is not used by anyone
until the the boot CPU is switching to the runtime page-tables.

In the future, the boot CPU will not switch between page-tables to
avoid TLB incoherency. Thus, the fixmap table will need to be always
hooked beofre any use. Let's start doing it now in setup_fixmap().

Lastly, document the behavior and the main registers usage within the
function.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

---
    Changes in v3:
        - The unused label is now removed in the previous patch
        - Add Stefano's reviewed-by

    Changes in v2:
        - Patch added
---
 xen/arch/arm/arm32/head.S | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)
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Patch

diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
index 34fc9ffcee..ee5c3d2af8 100644
--- a/xen/arch/arm/arm32/head.S
+++ b/xen/arch/arm/arm32/head.S
@@ -541,8 +541,21 @@  identity_mapping_removed:
         mov   pc, lr
 ENDPROC(remove_identity_mapping)
 
+/*
+ * Map the UART in the fixmap (when earlyprintk is used) and hook the
+ * fixmap table in the page tables.
+ *
+ * The fixmap cannot be mapped in create_page_tables because it may
+ * clash with the 1:1 mapping.
+ *
+ * Inputs:
+ *   r10: Physical offset
+ *   r11: Early UART base physical address
+ *
+ * Clobbers r1 - r4
+ */
 setup_fixmap:
-#if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */
+#if defined(CONFIG_EARLY_PRINTK)
         /* Add UART to the fixmap table */
         ldr   r1, =xen_fixmap        /* r1 := vaddr (xen_fixmap) */
         lsr   r2, r11, #THIRD_SHIFT
@@ -551,6 +564,7 @@  setup_fixmap:
         orr   r2, r2, #PT_LOWER(DEV_L3) /* r2:r3 := 4K dev map including UART */
         mov   r3, #0x0
         strd  r2, r3, [r1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's slot */
+#endif
 
         /* Map fixmap into boot_second */
         ldr   r1, =boot_second       /* r1 := vaddr (boot_second) */
@@ -565,7 +579,7 @@  setup_fixmap:
 
         /* Ensure any page table updates made above have occurred. */
         dsb   nshst
-#endif
+
         mov   pc, lr
 ENDPROC(setup_fixmap)