在 2023/8/15 22:50, Sean Christopherson 写道: > On Tue, Aug 15, 2023, Yan Zhao wrote: >> On Mon, Aug 14, 2023 at 09:40:44AM -0700, Sean Christopherson wrote: >>>>> Note, I'm assuming secondary MMUs aren't allowed to map swap entries... >>>>> >>>>> Compile tested only. >>>> >>>> I don't find a matching end to each >>>> mmu_notifier_invalidate_range_start_nonblock(). >>> >>> It pairs with existing call to mmu_notifier_invalidate_range_end() in change_pmd_range(): >>> >>> if (range.start) >>> mmu_notifier_invalidate_range_end(&range); >> No, It doesn't work for mmu_notifier_invalidate_range_start() sent in change_pte_range(), >> if we only want the range to include pages successfully set to PROT_NONE. > > Precise invalidation was a non-goal for my hack-a-patch. The intent was purely > to defer invalidation until it was actually needed, but still perform only a > single notification so as to batch the TLB flushes, e.g. the start() call still > used the original @end. > > The idea was to play nice with the scenario where nothing in a VMA could be migrated. > It was complete untested though, so it may not have actually done anything to reduce > the number of pointless invalidations. For numa-balance scenery, can original page still be used by application even if pte is changed with PROT_NONE? If it can be used, maybe we can zap shadow mmu and flush tlb in notification mmu_notifier_invalidate_range_end with precised range, the range can be cross-range between range mmu_gather and mmu_notifier_range. Regards Bibo Mao > >>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c >>> index 9e4cd8b4a202..f29718a16211 100644 >>> --- a/arch/x86/kvm/mmu/mmu.c >>> +++ b/arch/x86/kvm/mmu/mmu.c >>> @@ -4345,6 +4345,9 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault, >>> if (unlikely(!fault->slot)) >>> return kvm_handle_noslot_fault(vcpu, fault, access); >>> >>> + if (mmu_invalidate_retry_hva(vcpu->kvm, fault->mmu_seq, fault->hva)) >>> + return RET_PF_RETRY; >>> + >> This can effectively reduce the remote flush IPIs a lot! >> One Nit is that, maybe rmb() or READ_ONCE() is required for kvm->mmu_invalidate_range_start >> and kvm->mmu_invalidate_range_end. >> Otherwise, I'm somewhat worried about constant false positive and retry. > > If anything, this needs a READ_ONCE() on mmu_invalidate_in_progress. The ranges > aren't touched when when mmu_invalidate_in_progress goes to zero, so ensuring they > are reloaded wouldn't do anything. The key to making forward progress is seeing > that there is no in-progress invalidation. > > I did consider adding said READ_ONCE(), but practically speaking, constant false > positives are impossible. KVM will re-enter the guest when retrying, and there > is zero chance of the compiler avoiding reloads across VM-Enter+VM-Exit. > > I suppose in theory we might someday differentiate between "retry because a different > vCPU may have fixed the fault" and "retry because there's an in-progress invalidation", > and not bother re-entering the guest for the latter, e.g. have it try to yield > instead. > > All that said, READ_ONCE() on mmu_invalidate_in_progress should effectively be a > nop, so it wouldn't hurt to be paranoid in this case. > > Hmm, at that point, it probably makes sense to add a READ_ONCE() for mmu_invalidate_seq > too, e.g. so that a sufficiently clever compiler doesn't completely optimize away > the check. Losing the check wouldn't be problematic (false negatives are fine, > especially on that particular check), but the generated code would *look* buggy.
