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SubjectRe: [PATCH v2] KVM: x86/MMU: Do not check unsync status for root SP.
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On 07/02/21 13:22, Yu Zhang wrote:
> In shadow page table, only leaf SPs may be marked as unsync.
> And for non-leaf SPs, we use unsync_children to keep the number
> of the unsynced children. In kvm_mmu_sync_root(), sp->unsync
> shall always be zero for the root SP, , hence no need to check
> it. Instead, a warning inside mmu_sync_children() is added, in
> case someone incorrectly used it.
>
> Also, clarify the mmu_need_write_protect(), by moving the warning
> into kvm_unsync_page().
>
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

This should really be more of a Co-developed-by, and there are a couple
adjustments that could be made in the commit message. I've queued the
patch and I'll fix it up later.

Paolo

> ---
> Changes in V2:
> - warnings added based on Sean's suggestion.
>
> 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 86af582..c4797a00cc 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(sp->unsync);
> +
> while (mmu_unsync_walk(parent, &pages)) {
> bool protected = false;
>
> @@ -2502,6 +2508,8 @@ int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)
>
> 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);
>

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