KVM: x86/mmu: Atomically clear SPTE dirty state in the clear-dirty-log flow

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Optimize the clearing of dirty state in TDP MMU SPTEs by doing an
atomic-AND (on SPTEs that have volatile bits) instead of the full XCHG
that currently ends up being invoked (see kvm_tdp_mmu_write_spte()).
Clearing _only_ the bit in question will allow KVM to skip the many
irrelevant checks in __handle_changed_spte() by avoiding any collateral
damage due to the XCHG writing all SPTE bits, e.g. the XCHG could race
with fast_page_fault() setting the W-bit and the CPU setting the D-bit,
and thus incorrectly drop the CPU's D-bit update.

Link: https://lore.kernel.org/all/Y9hXmz%2FnDOr1hQal@google.com
Signed-off-by: Vipin Sharma <vipinsh@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
[sean: split the switch to atomic-AND to a separate patch]
Link: https://lore.kernel.org/r/20230321220021.2119033-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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Vipin Sharma 2023-03-21 15:00:12 -07:00 • committed by Sean Christopherson

parent 697c89bed9

commit 89c313f20c

2 changed files with 22 additions and 8 deletions

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14

arch/x86/kvm/mmu/tdp_iter.h

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16

arch/x86/kvm/mmu/tdp_mmu.c

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