x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case

Since

  72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly reflect state when running on Xen")

PAT can be enabled without MTRR.

This has resulted in problems e.g. for a SEV-SNP guest running under Hyper-V,
when trying to establish a new mapping via memremap() with WB caching mode, as
pat_x_mtrr_type() will call mtrr_type_lookup(), which in turn is returning
MTRR_TYPE_INVALID due to MTRR being disabled in this configuration.

The result is a mapping with UC- caching, leading to severe performance
degradation.

Fix that by handling MTRR_TYPE_INVALID the same way as MTRR_TYPE_WRBACK
in pat_x_mtrr_type() because MTRR_TYPE_INVALID means MTRRs are disabled.

  [ bp: Massage commit message. ]

Fixes: 72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly reflect state when running on Xen")
Reported-by: Michael Kelley (LINUX) <mikelley@microsoft.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/r/20230110065427.20767-1-jgross@suse.com

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index 46de9cf5c91d2be26a72afdd4de86b87c2b2c3c2..fb4b1b5e0deab2fcbd1f1a2442431fe63adbbc24 100644 (file)
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -387,7 +387,8 @@ static unsigned long pat_x_mtrr_type(u64 start, u64 end,
                u8 mtrr_type, uniform;
 
                mtrr_type = mtrr_type_lookup(start, end, &uniform);
-               if (mtrr_type != MTRR_TYPE_WRBACK)
+               if (mtrr_type != MTRR_TYPE_WRBACK &&
+                   mtrr_type != MTRR_TYPE_INVALID)
                        return _PAGE_CACHE_MODE_UC_MINUS;
 
                return _PAGE_CACHE_MODE_WB;