Messages in this thread | | | From | "Michael Kelley (LINUX)" <> | Subject | RE: [PATCH] x86/pat: fix pat_x_mtrr_type() for MTRR disabled case | Date | Tue, 10 Jan 2023 15:22:31 +0000 |
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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>
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