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SubjectRe: [PATCH 1/2] KVM: x86/mmu: Protect marking SPs unsync when using TDP MMU with spinlock
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On 11/08/21 17:52, Sean Christopherson wrote:
> All that said, I do not have a strong preference. Were you thinking something
> like this?

Yes, pretty much this.

Paolo

> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index d574c68cbc5c..b622e8a13b8b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2595,6 +2595,7 @@ static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
> int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync)
> {
> struct kvm_mmu_page *sp;
> + bool locked = false;
>
> /*
> * Force write-protection if the page is being tracked. Note, the page
> @@ -2617,9 +2618,34 @@ int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync)
> if (sp->unsync)
> continue;
>
> + /*
> + * TDP MMU page faults require an additional spinlock as they
> + * run with mmu_lock held for read, not write, and the unsync
> + * logic is not thread safe. Take the spinklock regardless of
> + * the MMU type to avoid extra conditionals/parameters, there's
> + * no meaningful penalty if mmu_lock is held for write.
> + */
> + if (!locked) {
> + locked = true;
> + spin_lock(&kvm->arch.mmu_unsync_pages_lock);
> +
> + /*
> + * Recheck after taking the spinlock, a different vCPU
> + * may have since marked the page unsync. A false
> + * positive on the unprotected check above is not
> + * possible as clearing sp->unsync_must_ hold mmu_lock
> + * for write, i.e. unsync cannot transition from 0->1
> + * while this CPU holds mmu_lock for read.
> + */
> + if (READ_ONCE(sp->unsync))
> + continue;
> + }
> +
> WARN_ON(sp->role.level != PG_LEVEL_4K);
> kvm_unsync_page(vcpu, sp);
> }
> + if (locked)
> + spin_unlock(&kvm->arch.mmu_unsync_pages_lock);

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