linux-kernel - [RFC PATCH 0/9] KVM: x86/mmu: Preserve Accessed bits on PROT changes

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Message-ID: <20240801183453.57199-1-seanjc@google.com>
Date: Thu,  1 Aug 2024 11:34:44 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 0/9] KVM: x86/mmu: Preserve Accessed bits on PROT changes

This applies on top of the massive "follow pfn" rework[*].  The gist is to
avoid losing accessed information, e.g. because NUMA balancing mucks with
PTEs, by preserving accessed state when KVM zaps SPTEs in response to
mmu_notifier invalidations that are for protection changes, e.g. PROT_NUMA.

RFC as I haven't done any testing to verify whether or not this has any
impact on page aging, let alone has _postivie_ impact.  Personally, I'm not
at all convinced that this is necessary outside of tests that care about
exact counts, e.g. KVM selftests.

That said, I do think patches 1-7 would be worth merging on their own.
Using A/D bits to track state even when A/D bits are disabled in hardware
is a nice cleanup.

[*] https://lore.kernel.org/all/20240726235234.228822-1-seanjc@google.com

Sean Christopherson (9):
  KVM: x86/mmu: Add a dedicated flag to track if A/D bits are globally
    enabled
  KVM: x86/mmu: Set shadow_accessed_mask for EPT even if A/D bits
    disabled
  KVM: x86/mmu: Set shadow_dirty_mask for EPT even if A/D bits disabled
  KVM: x86/mmu: Use Accessed bit even when _hardware_ A/D bits are
    disabled
  KVM: x86/mmu: Free up A/D bits in FROZEN_SPTE
  KVM: x86/mmu: Process only valid TDP MMU roots when aging a gfn range
  KVM: x86/mmu: Stop processing TDP MMU roots for test_age if young SPTE
    found
  KVM: Plumb mmu_notifier invalidation event type into arch code
  KVM: x86/mmu: Track SPTE accessed info across mmu_notifier PROT
    changes

 arch/x86/kvm/mmu/mmu.c     |  10 ++--
 arch/x86/kvm/mmu/spte.c    |  16 ++++--
 arch/x86/kvm/mmu/spte.h    |  39 +++++--------
 arch/x86/kvm/mmu/tdp_mmu.c | 113 +++++++++++++++++++++----------------
 include/linux/kvm_host.h   |   1 +
 virt/kvm/kvm_main.c        |   1 +
 6 files changed, 99 insertions(+), 81 deletions(-)

base-commit: 93a198738e0aeb3193ca39c9f01f66060b3c4910
-- 
2.46.0.rc1.232.g9752f9e123-goog