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SubjectRe: [PATCH v3] KVM: x86/MMU: Do not check unsync status for root SP.
Sorry, forget the change log:

Changes in V3:
- fixed a bug in warnings inside mmu_sync_children().
- commit message changes based on Paolo's suggestion.
- added Co-developed-by: Sean Christopherson <seanjc@google.com>

Changes in V2:
- warnings added based on Sean's suggestion.


On Wed, Feb 10, 2021 at 01:01:11AM +0800, Yu Zhang wrote:
> In shadow page table, only leaf SPs may be marked as unsync;
> instead, for non-leaf SPs, we store the number of unsynced
> children in unsync_children. Therefore, in kvm_mmu_sync_root(),
> sp->unsync shall always be zero for the root SP and there is
> no need to check it. Remove the check, and add a warning
> inside mmu_sync_children() to assert that the flags are used
> properly.
>
> While at it, move the warning from mmu_need_write_protect()
> to kvm_unsync_page().
>
> Co-developed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 86af58294272..5f482af125b4 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1995,6 +1995,12 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
> LIST_HEAD(invalid_list);
> bool flush = false;
>
> + /*
> + * Only 4k SPTEs can directly be made unsync, the parent pages
> + * should never be unsyc'd.
> + */
> + WARN_ON_ONCE(parent->unsync);
> +
> while (mmu_unsync_walk(parent, &pages)) {
> bool protected = false;
>
> @@ -2502,6 +2508,8 @@ EXPORT_SYMBOL_GPL(kvm_mmu_unprotect_page);
>
> static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
> {
> + WARN_ON(sp->role.level != PG_LEVEL_4K);
> +
> trace_kvm_mmu_unsync_page(sp);
> ++vcpu->kvm->stat.mmu_unsync;
> sp->unsync = 1;
> @@ -2524,7 +2532,6 @@ bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
> if (sp->unsync)
> continue;
>
> - WARN_ON(sp->role.level != PG_LEVEL_4K);
> kvm_unsync_page(vcpu, sp);
> }
>
> @@ -3406,8 +3413,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
> * mmu_need_write_protect() describe what could go wrong if this
> * requirement isn't satisfied.
> */
> - if (!smp_load_acquire(&sp->unsync) &&
> - !smp_load_acquire(&sp->unsync_children))
> + if (!smp_load_acquire(&sp->unsync_children))
> return;
>
> write_lock(&vcpu->kvm->mmu_lock);
> --
> 2.17.1
>

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