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KVM: x86: Further clarify the logic and comments for toggling log dirty 

Add a sanity check in kvm_mmu_slot_apply_flags to assert that the
LOG_DIRTY_PAGES flag is indeed being toggled, and explicitly rely on
that holding true when zapping collapsible SPTEs.  Manipulating the
CPU dirty log (PML) and write-protection also relies on this assertion,
but that's not obvious in the current code.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210213005015.1651772-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2021-02-12 16:50:11 -08:00 committed by Paolo Bonzini
parent a018eba538
commit 52f4607940
1 changed files with 11 additions and 4 deletions
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15
arch/x86/kvm/x86.c
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@ -10761,12 +10761,20 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
enum kvm_mr_change change)
{
/*
* Nothing to do for RO slots or CREATE/MOVE/DELETE of a slot.
* See comments below.
* Nothing to do for RO slots (which can't be dirtied and can't be made
* writable) or CREATE/MOVE/DELETE of a slot. See comments below.
*/
if ((change != KVM_MR_FLAGS_ONLY) || (new->flags & KVM_MEM_READONLY))
return;
/*
* READONLY and non-flags changes were filtered out above, and the only
* other flag is LOG_DIRTY_PAGES, i.e. something is wrong if dirty
* logging isn't being toggled on or off.
*/
if (WARN_ON_ONCE(!((old->flags ^ new->flags) & KVM_MEM_LOG_DIRTY_PAGES)))
return;
/*
* Dirty logging tracks sptes in 4k granularity, meaning that large
* sptes have to be split. If live migration is successful, the guest
@ -10784,8 +10792,7 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
* MOVE/DELETE: The old mappings will already have been cleaned up by
* kvm_arch_flush_shadow_memslot()
*/
if ((old->flags & KVM_MEM_LOG_DIRTY_PAGES) &&
!(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
if (!(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
kvm_mmu_zap_collapsible_sptes(kvm, new);
/*