Messages in this thread | | | Subject | Re: [PATCH 1/2] KVM: x86/mmu: Protect marking SPs unsync when using TDP MMU with spinlock | From | Paolo Bonzini <> | Date | Thu, 12 Aug 2021 17:37:55 +0200 |
| |
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);
| |