linux-kernel - Re: [PATCH 0/4] KVM: x86/mmu: Memtype related cleanups

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Date:   Tue, 19 Jul 2022 19:59:58 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] KVM: x86/mmu: Memtype related cleanups

On 7/16/22 01:00, Sean Christopherson wrote:
> Minor cleanups for KVM's handling of the memtype that's shoved into SPTEs.
> 
> Patch 1 enforces that entry '0' of the host's IA32_PAT is configured for WB
> memtype.  KVM subtle relies on this behavior (silently shoves '0' into the
> SPTE PAT field).  Check this at KVM load time so that if that doesn't hold
> true, KVM will refuse to load instead of running the guest with weird and
> potentially dangerous memtypes.
> 
> Patch 2 is a pure code cleanup (ordered after patch 1 in case someone wants
> to backport the PAT check).
> 
> Patch 3 add a mask to track whether or not KVM may use a non-zero memtype
> value in SPTEs.  Essentially, it's a "is EPT enabled" flag without being an
> explicit "is EPT enabled" flag.  This avoid some minor work when not using
> EPT, e.g. technically KVM could drop the RET0 implemention that's used for
> SVM's get_mt_mask(), but IMO that's an unnecessary risk.
> 
> Patch 4 modifies the TDP page fault path to restrict the mapping level
> based on guest MTRRs if and only if KVM might actually consume them.  The
> guest MTRRs are purely software constructs (not directly consumed by
> hardware), and KVM only honors them when EPT is enabled (host MTRRs are
> overridden by EPT) and the guest has non-coherent DMA.  I doubt this will
> move the needed on whether or not KVM can create huge pages, but it does
> save having to do MTRR lookups on every page fault for guests without
> a non-coherent DMA device attached.
> 
> Sean Christopherson (4):
>    KVM: x86: Reject loading KVM if host.PAT[0] != WB
>    KVM: x86: Drop unnecessary goto+label in kvm_arch_init()
>    KVM: x86/mmu: Add shadow mask for effective host MTRR memtype
>    KVM: x86/mmu: Restrict mapping level based on guest MTRR iff they're
>      used
> 
>   arch/x86/kvm/mmu/mmu.c  | 26 +++++++++++++++++++-------
>   arch/x86/kvm/mmu/spte.c | 21 ++++++++++++++++++---
>   arch/x86/kvm/mmu/spte.h |  1 +
>   arch/x86/kvm/x86.c      | 33 ++++++++++++++++++++-------------
>   4 files changed, 58 insertions(+), 23 deletions(-)
> 
> 
> base-commit: 8031d87aa9953ddeb047a5356ebd0b240c30f233

Queued, thanks.

Paolo