RE: [PATCH 0/6] x86/mtrr: fix handling with PAT but without MTRR

[Michael Kelley (LINUX)]

From: Juergen Gross <jgross@xxxxxxxx> Sent: Monday, February 6, 2023 11:29 PM
> 
> This series tries to fix the rather special case of PAT being available
> without having MTRRs (either due to CONFIG_MTRR being not set, or
> because the feature has been disabled e.g. by a hypervisor).
> 
> The main use cases are Xen PV guests and SEV-SNP guests running under
> Hyper-V.
> 
> Patch 2 seems to be a little hacky, as it special cases only
> memtype_reserve() and memtype_free(), but OTOH this doesn't seem to
> be worse than in previous days, where PAT was disabled when MTRRs
> haven't been available.
> 
> My tests with Xen didn't show any problems, but I'm rather sure I
> couldn't cover all corner cases.

I tested this patch set with Hyper-V SEV-SNP guests, and ioremap_cache()
is correctly mapping as WB.

As an observation, with commit 90b926e68f50 it was nice to have
the memtype entries created.  I could check for any unexpected
mappings in /sys/kernel/debug/x86/pat_memtype_list.  With this patch
set, we're back to not creating those entries.  

Michael

> 
> The only cleaner solution I could think of would be to introduce MTRR
> read-only access. It would theoretically be possible to get the actual
> MTRR contents for the variable MTRRs from Xen, but I'm not sure this
> is really the way to go.
> 
> For the SEV-SNP case with Hyper-V I guess such a read-only mode could
> be rather simple, but I'm really not sure this would cover all needed
> corner cases (I'd basically say always "WB" in that case).
> 
> I have added more cleanup which has been discussed when looking into
> the most recent failures.
> 
> Juergen Gross (6):
>   x86/mtrr: make mtrr_enabled() non-static
>   x86/pat: check for MTRRs enabled in memtype_reserve()
>   x86/mtrr: revert commit 90b926e68f50
>   x86/mtrr: don't let mtrr_type_lookup() return MTRR_TYPE_INVALID
>   x86/mm: only check uniform after calling mtrr_type_lookup()
>   x86/mtrr: drop sanity check in mtrr_type_lookup_fixed()
> 
>  arch/x86/include/asm/mtrr.h        | 13 +++++++++++--
>  arch/x86/include/uapi/asm/mtrr.h   |  6 +++---
>  arch/x86/kernel/cpu/mtrr/generic.c | 10 +++-------
>  arch/x86/kernel/cpu/mtrr/mtrr.c    |  2 +-
>  arch/x86/mm/pat/memtype.c          | 13 ++++++++-----
>  arch/x86/mm/pgtable.c              |  6 ++----
>  6 files changed, 28 insertions(+), 22 deletions(-)
> 
> --
> 2.35.3