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Re: [Xen-devel] [PATCH v2 32/35] xen/arm32: head: Rework and document setup_fixmap()



Hi Stefano,

On 7/30/19 10:14 PM, Stefano Stabellini wrote:
On Mon, 22 Jul 2019, Julien Grall wrote:
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@xxxxxxx>

---
     Changes in v2:
         - Patch added
---
  xen/arch/arm/arm32/head.S | 19 ++++++++++++++++---
  1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
index 56e2d09ed4..e0f8c2e0cb 100644
--- a/xen/arch/arm/arm32/head.S
+++ b/xen/arch/arm/arm32/head.S
@@ -536,8 +536,21 @@ 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
@@ -546,7 +559,7 @@ 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 */
-1:
+#endif

Patch is OK. However, the 1: should be removed in the previous patch
"xen/arm32: head: Don't setup the fixmap on secondary CPUs", where we
took away the beq.

Good point. I have now moved this to the previous patch.


Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

Thank you!

Cheers,

--
Julien Grall

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