[Xen-devel] [PATCH] x86/boot: Don't explicitly map the VGA region as UC-

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

All 64-bit capable processors use PAT, and with PAT, it is explicitly
permitted to have mappings with a cacheability different to MTRRs.

On a native system with a real legacy VGA region, MTRRs will cause the region
to be UC-.  When booting virtualised, this range may be RAM and not a legacy
VGA region, at which point it wants to be WB.

Use WB mapping in the pagetables, such that in systems without a legacy VGA
region, the RAM between 0xa0000 and 0xc0000 doesn't become unnecesserily UC-.
However, we still must use small pages to avoid the undefined behaviour caused
by superpages crossing MTRRs of different cacheability.

While adjusting the pagetable construction, switch from pfn to idx for
consistency with all the other construction logic.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

As a future optimisation, Xen could rewrite l2_identmap with a superpage if it
finds that MTRRs are disabled.  However, this probably needs to wait until Xen
no longer unconditionally assumes a legacy VGA region to be present in the
e820 if it finds something other than a hole
---
 xen/arch/x86/boot/x86_64.S | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index 5ab24d73fc..1ca986f882 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -51,19 +51,13 @@ GLOBAL(stack_start)
 /*
  * Mapping of first 2 megabytes of memory. This is mapped with 4kB mappings
  * to avoid type conflicts with fixed-range MTRRs covering the lowest megabyte
- * of physical memory. In any case the VGA hole should be mapped with type UC.
- * Uses 1x 4k page.
+ * of physical memory.  Uses 1x 4k page.
  */
 l1_identmap:
-        pfn = 0
+        idx = 0
         .rept L1_PAGETABLE_ENTRIES
-        /* VGA hole (0xa0000-0xc0000) should be mapped UC-. */
-        .if pfn >= 0xa0 && pfn < 0xc0
-        .quad (pfn << PAGE_SHIFT) | PAGE_HYPERVISOR_UCMINUS | MAP_SMALL_PAGES
-        .else
-        .quad (pfn << PAGE_SHIFT) | PAGE_HYPERVISOR | MAP_SMALL_PAGES
-        .endif
-        pfn = pfn + 1
+        .quad (idx << PAGE_SHIFT) | PAGE_HYPERVISOR | MAP_SMALL_PAGES
+        idx = idx + 1
         .endr
         .size l1_identmap, . - l1_identmap
 
-- 
2.11.0


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