Since commit 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().

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>
---
 arch/x86/mm/pat/memtype.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index 46de9cf5c91d..fb4b1b5e0dea 100644
--- 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;
-- 
2.35.3

From: Juergen Gross <jgross@suse.com> Sent: Monday, January 9, 2023 10:54 PM
> 
> Since commit 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().
> 
> 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>
> ---
>  arch/x86/mm/pat/memtype.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> index 46de9cf5c91d..fb4b1b5e0dea 100644
> --- 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;
> --
> 2.35.3

This looks good to me.  I've tested my specific use case of a SEV-SNP guest
running under Hyper-V, where the MTRRs are not visible in the guest but the
PAT is functional.  Previously, this use case showed many "uncached-minus"
entries in /sys/kernel/debug/x86/pat_memtype_list even though the mapping
requests were made as WB.  With this patch, those entries show "write-back",
as expected.

Thanks!

Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Michael Kelley <mikelley@microsoft.com>

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     90b926e68f500844dff16b5bcea178dc55cf580a
Gitweb:        https://git.kernel.org/tip/90b926e68f500844dff16b5bcea178dc55cf580a
Author:        Juergen Gross <jgross@suse.com>
AuthorDate:    Tue, 10 Jan 2023 07:54:27 +01:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Tue, 10 Jan 2023 17:21:53 +01:00

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
---
 arch/x86/mm/pat/memtype.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index 46de9cf..fb4b1b5 100644
--- 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;