Messages in this thread | | | From | "Maciej S. Szmigiero" <> | Subject | Re: [PATCH 2/3] KVM: x86/mmu: Drop a redundant remote TLB flush in kvm_zap_gfn_range() | Date | Tue, 26 Oct 2021 16:22:32 +0200 |
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On 25.10.2021 17:39, Sean Christopherson wrote: > On Fri, Oct 22, 2021, Maciej S. Szmigiero wrote: >> On 22.10.2021 03:00, Sean Christopherson wrote: >>> Remove an unnecessary remote TLB flush in kvm_zap_gfn_range() now that >>> said function holds mmu_lock for write for its entire duration. The >>> flush was added by the now-reverted commit to allow TDP MMU to flush while >>> holding mmu_lock for read, as the transition from write=>read required >>> dropping the lock and thus a pending flush needed to be serviced. >>> >>> Fixes: 5a324c24b638 ("Revert "KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock"") >>> Cc: Maxim Levitsky <mlevitsk@redhat.com> >>> Cc: Maciej S. Szmigiero <maciej.szmigiero@oracle.com> >>> Cc: Ben Gardon <bgardon@google.com> >>> Signed-off-by: Sean Christopherson <seanjc@google.com> >>> --- >>> arch/x86/kvm/mmu/mmu.c | 3 --- >>> 1 file changed, 3 deletions(-) >>> >>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c >>> index f82b192bba0b..e8b8a665e2e9 100644 >>> --- a/arch/x86/kvm/mmu/mmu.c >>> +++ b/arch/x86/kvm/mmu/mmu.c >>> @@ -5700,9 +5700,6 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end) >>> end - 1, true, flush); >>> } >>> } >>> - if (flush) >>> - kvm_flush_remote_tlbs_with_address(kvm, gfn_start, >>> - gfn_end - gfn_start); >>> } >>> if (is_tdp_mmu_enabled(kvm)) { >>> >> >> Unfortunately, it seems that a pending flush from __kvm_zap_rmaps() >> can be reset back to false by the following line: >>> flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, gfn_start, gfn_end, flush); >> >> kvm_tdp_mmu_zap_gfn_range() calls __kvm_tdp_mmu_zap_gfn_range with >> "can_yield" set to true, which passes it to zap_gfn_range, which has >> this code: >>> if (can_yield && >>> tdp_mmu_iter_cond_resched(kvm, &iter, flush, shared)) { >>> flush = false; >>> continue; >>> } > > That's working by design. If the MMU (legacy or TDP) yields during zap, it _must_ > flush before dropping mmu_lock so that any SPTE modifications are guaranteed to be > observed by all vCPUs. Clearing "flush" is deliberate/correct as another is flush > is needed if and only if additional SPTE modifications are made. >
Got it, thanks.
Maciej
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