[PATCH v4 02/10] x86/mm: Avoid hard-coding PAT in get_page_from_l1e()

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

get_page_from_l1e() relied on Xen's choice of PAT, which is brittle in
the face of future PAT changes.  Instead, compute the actual cacheability
used by the CPU and switch on that, as this will work no matter what PAT
Xen uses.

No functional change intended.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
---
Changes since v3:
- Compute and use the actual cacheability as seen by the processor.

Changes since v2:
- Improve commit message.
---
 xen/arch/x86/include/asm/processor.h |  8 ++++++++
 xen/arch/x86/mm.c                    | 19 +++++++++++++------
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/include/asm/processor.h 
b/xen/arch/x86/include/asm/processor.h
index 
8e2816fae9b97bd4e153a30cc3802971fe0355af..c3205ed9e10c1a57d23c5ecead66bebd82d87d06
 100644
--- a/xen/arch/x86/include/asm/processor.h
+++ b/xen/arch/x86/include/asm/processor.h
@@ -100,6 +100,14 @@
 
 #ifndef __ASSEMBLY__
 
+/* Convert from PAT/PCD/PWT embedded in PTE flags to actual cacheability value 
*/
+static inline unsigned int pte_flags_to_cacheability(unsigned int flags)
+{
+    unsigned int pat_shift = ((flags & _PAGE_PAT) >> 2) |
+                             (flags & (_PAGE_PCD|_PAGE_PWT));
+    return 0xFF & (XEN_MSR_PAT >> pat_shift);
+}
+
 struct domain;
 struct vcpu;
 
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 
78b1972e4170cacccc9c37c6e64e76e66a7da87f..802073a01c5cf4dc3cf1d58d28ea4d4e9e8149c7
 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -959,15 +959,22 @@ get_page_from_l1e(
             flip = _PAGE_RW;
         }
 
-        switch ( l1f & PAGE_CACHE_ATTRS )
+        /* Force cacheable memtypes to UC */
+        switch ( pte_flags_to_cacheability(l1f) )
         {
-        case 0: /* WB */
-            flip |= _PAGE_PWT | _PAGE_PCD;
+        case X86_MT_UC:
+        case X86_MT_UCM:
+        case X86_MT_WC:
+            /* not cached */
             break;
-        case _PAGE_PWT: /* WT */
-        case _PAGE_PWT | _PAGE_PAT: /* WP */
-            flip |= _PAGE_PCD | (l1f & _PAGE_PAT);
+        case X86_MT_WB:
+        case X86_MT_WT:
+        case X86_MT_WP:
+            /* cacheable, force to UC */
+            flip |= (l1f & PAGE_CACHE_ATTRS) ^ _PAGE_UC;
             break;
+        default:
+            BUG();
         }
 
         return flip;
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab