[Xen-devel] [PATCH 15/17] xen/arm64: head: Rework and document setup_fixmap()

[Julien Grall <julien.grall@xxxxxxx>]

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 conflict. This means the fixmap table will need to be hooked before
any use. For simplicity, setup_fixmap() will now do that job.

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

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
---
 xen/arch/arm/arm64/head.S | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 96e85f8834..4f7fa6769f 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -700,8 +700,17 @@ id_map_removed:
         ret
 ENDPROC(remove_id_map)
 
+/*
+ * 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 ID map.
+ *
+ * Clobbers x0 - x1
+ */
 setup_fixmap:
-#if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */
+#ifdef CONFIG_EARLY_PRINTK
         /* Add UART to the fixmap table */
         ldr   x1, =xen_fixmap        /* x1 := vaddr (xen_fixmap) */
         lsr   x2, x23, #THIRD_SHIFT
@@ -709,6 +718,7 @@ setup_fixmap:
         mov   x3, #PT_DEV_L3
         orr   x2, x2, x3             /* x2 := 4K dev map including UART */
         str   x2, [x1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's 
slot */
+#endif
 
         /* Map fixmap into boot_second */
         ldr   x4, =boot_second       /* x4 := vaddr (boot_second) */
@@ -721,7 +731,6 @@ setup_fixmap:
 
         /* Ensure any page table updates made above have occurred */
         dsb   nshst
-#endif
         ret
 ENDPROC(setup_fixmap)
 
-- 
2.11.0


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