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AJvYcCUge8dmnWwzfVT7Hq/1pc97cEWl16YxCQ6C6KtYFjtAeb4tSnzXvv0HPP2vs/lxSQfIrMA=@vger.kernel.org X-Gm-Message-State: AOJu0YzozFwcNH4Sir63o/ve9dKB3a31piigfNQn2CYTZ+KncIA4gqV7 OymE7dwDdopSkSOLlhuE3Rq1tWrbxrN8iJOlMg8jHfr8oen3VyWalbyYyoojI/3Kq4NshB5pJ0I 5+HQvYQ== X-Google-Smtp-Source: AGHT+IEm4/q7R9vqsalDGxIoUCdP69IoDYNKhvzEhXQffHVAOixTwa8x+rJdf1yKoahs9B9e41d388NCl58= X-Received: from pjn5.prod.google.com ([2002:a17:90b:5705:b0:325:b894:3c4f]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:384a:b0:325:8e03:c159 with SMTP id 98e67ed59e1d1-3258e03c39amr6971375a91.9.1756155924087; Mon, 25 Aug 2025 14:05:24 -0700 (PDT) Date: Mon, 25 Aug 2025 14:05:22 -0700 In-Reply-To: <20250822080235.27274-1-yan.y.zhao@intel.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250822080100.27218-1-yan.y.zhao@intel.com> <20250822080235.27274-1-yan.y.zhao@intel.com> Message-ID: Subject: Re: [PATCH v3 2/3] KVM: Skip invoking shared memory handler for entirely private GFN ranges From: Sean Christopherson To: Yan Zhao Cc: pbonzini@redhat.com, peterx@redhat.com, rick.p.edgecombe@intel.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Content-Type: text/plain; charset="us-ascii" On Fri, Aug 22, 2025, Yan Zhao wrote: > When a GFN range is entirely private, it's unnecessary for > kvm_handle_hva_range() to invoke handlers for the GFN range, because > 1) the gfn_range.attr_filter for the handler is KVM_FILTER_SHARED, which > is for shared mappings only; > 2) KVM has already zapped all shared mappings before setting the memory > attribute to private. > > This can avoid unnecessary zaps on private mappings for VMs of type > KVM_X86_SW_PROTECTED_VM, e.g., during auto numa balancing scans of VMAs. This feels like the wrong place to try and optimize spurious zaps. x86 should be skipping SPTEs that don't match. For KVM_X86_SW_PROTECTED_VM, I don't think we care about spurious zpas, because that's a testing-only type that doesn't have line of sight to be being a "real" type. For SNP, we might care? But actually zapping private SPTEs would require userspace to retain the shared mappings across a transition, _and_ be running NUMA autobalancing in the first place. If someone actually cares about optimizing this scenario, KVM x86 could track private SPTEs via a software-available bit. We also want to move away from KVM_MEMORY_ATTRIBUTE_PRIVATE and instead track private vs. shared in the gmem instance. So I'm inclined to skip this... > Signed-off-by: Yan Zhao > --- > virt/kvm/kvm_main.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index f769d1dccc21..e615ad405ce4 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -620,6 +620,17 @@ static __always_inline kvm_mn_ret_t kvm_handle_hva_range(struct kvm *kvm, > gfn_range.slot = slot; > gfn_range.lockless = range->lockless; > > +#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES > + /* > + * If GFN range are all private, no need to invoke the > + * handler. > + */ > + if (kvm_range_has_memory_attributes(kvm, gfn_range.start, > + gfn_range.end, ~0, > + KVM_MEMORY_ATTRIBUTE_PRIVATE)) > + continue; > +#endif > + > if (!r.found_memslot) { > r.found_memslot = true; > if (!range->lockless) { > -- > 2.43.2 >