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Fri, 28 Feb 2025 18:17:54 -0800 (PST) X-Google-Smtp-Source: AGHT+IFN/iXX9LAQdg+hriAADmgoksNH2mZe0QdzYNTYyVp1fDU0v/SNpA9L9q5xpDeWRO8auI7xsA== X-Received: by 2002:a05:622a:244:b0:471:c999:2b81 with SMTP id d75a77b69052e-474bc09fe97mr61531481cf.29.1740795473756; Fri, 28 Feb 2025 18:17:53 -0800 (PST) Received: from starship ([2607:fea8:fc01:8d8d:6adb:55ff:feaa:b156]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-47472409a83sm31618611cf.55.2025.02.28.18.17.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 28 Feb 2025 18:17:53 -0800 (PST) Message-ID: Subject: Re: [RFC PATCH 11/13] KVM: nSVM: Do not reset TLB_CONTROL in VMCB02 on nested entry From: Maxim Levitsky To: Yosry Ahmed , Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org Date: Fri, 28 Feb 2025 21:17:52 -0500 In-Reply-To: <20250205182402.2147495-12-yosry.ahmed@linux.dev> References: <20250205182402.2147495-1-yosry.ahmed@linux.dev> <20250205182402.2147495-12-yosry.ahmed@linux.dev> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 7bit On Wed, 2025-02-05 at 18:24 +0000, Yosry Ahmed wrote: > TLB_CONTROL is reset to TLB_CONTROL_DO_NOTHING on nested transitions to > L2. This is unnecessary because it should always be > TLB_CONTROL_DO_NOTHING at this point. > > The flow for setting TLB_CONTROL is as follows: > 1. In vcpu_enter_guest(), servicing a TLB flush request may set it to > TLB_CONTROL_FLUSH_ASID in svm_flush_tlb_asid(). > 2. In svm_vcpu_run() -> pre_svm_run(), it may get upgraded to > TLB_CONTROL_FLUSH_ALL_ASID when assigning a new ASID. > 3. In svm_cpu_run(), it gets reset to TLB_CONTROL_DO_NOTHING after the > guest is run. > > Hence, TLB_CONTROL is reset after each run and there is no need to do it > again on every nested transition to L2. > > There is a TODO in nested_svm_transition_tlb_flush() about this reset > crushing pending TLB flushes. Remove it, as the reset is not really > crushing anything as explained above. I am not sure that we don't crush a pending tlb request: svm_flush_tlb_asid can also be called by KVM_REQ_TLB_FLUSH and set the flush request in both vmcbs, thus later the nested_svm_exit_tlb_flush can crush this request. But the patch itself does look OK to me, although I might be mistaken. Reviewed-by: Maxim Levitsky Best regards, Maxim Levitsky > > Signed-off-by: Yosry Ahmed > --- > arch/x86/kvm/svm/nested.c | 12 ++---------- > 1 file changed, 2 insertions(+), 10 deletions(-) > > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c > index 12bb391884299..8e40ff21f7353 100644 > --- a/arch/x86/kvm/svm/nested.c > +++ b/arch/x86/kvm/svm/nested.c > @@ -512,12 +512,7 @@ static void nested_svm_entry_tlb_flush(struct kvm_vcpu *vcpu) > svm->nested.last_asid = svm->nested.ctl.asid; > kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu); > } > - /* > - * TODO: optimize unconditional TLB flush/MMU sync. A partial list of > - * things to fix before this can be conditional: > - * > - * - Don't crush a pending TLB flush in vmcb02 on nested VMRUN > - */ > + /* TODO: optimize unconditional TLB flush/MMU sync */ > kvm_make_request(KVM_REQ_MMU_SYNC, vcpu); > kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu); > } > @@ -536,7 +531,7 @@ static void nested_svm_exit_tlb_flush(struct kvm_vcpu *vcpu) > if (svm->nested.ctl.tlb_ctl == TLB_CONTROL_FLUSH_ALL_ASID) > kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu); > > - /* See nested_svm_entry_tlb_flush() */ > + /* TODO: optimize unconditional TLB flush/MMU sync */ > kvm_make_request(KVM_REQ_MMU_SYNC, vcpu); > kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu); > } > @@ -717,9 +712,6 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm, > > /* Done at vmrun: asid. */ > > - /* Also overwritten later if necessary. */ > - svm_clear_tlb_ctl_flush(vmcb02); > - > /* nested_cr3. */ > if (nested_npt_enabled(svm)) > nested_svm_init_mmu_context(vcpu);