[PATCH v6 1/5] 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.  This code is itself questionable and may
be removed in the future, but removing it would be an observable
behavior change and so is out of scope for this patch series.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Changes since v5:
- Make comment in get_page_from_l1e() future-proof.
- Explicitly state how known-uncacheable and potentially-cacheable types
  are handled.

Changes since v4:
- Do not add new pte_flags_to_cacheability() helper, as this code may be
  removed in the near future and so adding a new helper for it is a bad
  idea.
- Do not BUG() in the event of an unexpected cacheability.  This cannot
  happen, but it is simpler to force such types to UC than to prove that
  the BUG() is not reachable.

Changes since v3:
- Compute and use the actual cacheability as seen by the processor.

Changes since v2:
- Improve commit message.
---
 xen/arch/x86/mm.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 
1bda1ba697b434b6c884f17e599aa9b6d3b3dd76..3558ca215b02a517d55d75329d645ae5905424e4
 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -959,14 +959,16 @@ get_page_from_l1e(
             flip = _PAGE_RW;
         }
 
-        switch ( l1f & PAGE_CACHE_ATTRS )
+        switch ( 0xFF & (XEN_MSR_PAT >> (8 * pte_flags_to_cacheattr(l1f))) )
         {
-        case 0: /* WB */
-            flip |= _PAGE_PWT | _PAGE_PCD;
+        case X86_MT_UC:
+        case X86_MT_UCM:
+        case X86_MT_WC:
+            /* not cacheable, allow */
             break;
-        case _PAGE_PWT: /* WT */
-        case _PAGE_PWT | _PAGE_PAT: /* WP */
-            flip |= _PAGE_PCD | (l1f & _PAGE_PAT);
+        default:
+            /* potentially cacheable, force to UC */
+            flip |= ((l1f & PAGE_CACHE_ATTRS) ^ _PAGE_UC);
             break;
         }
 
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab