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) <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>
---
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 <[email protected]> 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) <[email protected]>
> Signed-off-by: Juergen Gross <[email protected]>
> ---
> 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 <[email protected]>
Tested-by: Michael Kelley <[email protected]>
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 <[email protected]>
AuthorDate: Tue, 10 Jan 2023 07:54:27 +01:00
Committer: Borislav Petkov (AMD) <[email protected]>
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) <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Reviewed-by: Michael Kelley <[email protected]>
Tested-by: Michael Kelley <[email protected]>
Cc: <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
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;