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[89.176.161.20]) by smtp.gmail.com with ESMTPSA id 39sm42348076wrc.45.2019.08.26.23.41.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 26 Aug 2019 23:41:45 -0700 (PDT) From: Vitaly Kuznetsov To: lantianyu1986@gmail.com Cc: Tianyu Lan , kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, pbonzini@redhat.com, rkrcmar@redhat.com, corbet@lwn.net, kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com, sashal@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org, michael.h.kelley@microsoft.com Subject: Re: [PATCH V3 0/3] KVM/Hyper-V: Add Hyper-V direct tlb flush support In-Reply-To: <20190819131737.26942-1-Tianyu.Lan@microsoft.com> References: <20190819131737.26942-1-Tianyu.Lan@microsoft.com> Date: Tue, 27 Aug 2019 08:41:43 +0200 Message-ID: <87ftlnm7o8.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-hyperv-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org lantianyu1986@gmail.com writes: > From: Tianyu Lan > > This patchset is to add Hyper-V direct tlb support in KVM. Hyper-V > in L0 can delegate L1 hypervisor to handle tlb flush request from > L2 guest when direct tlb flush is enabled in L1. > > Patch 2 introduces new cap KVM_CAP_HYPERV_DIRECT_TLBFLUSH to enable > feature from user space. User space should enable this feature only > when Hyper-V hypervisor capability is exposed to guest and KVM profile > is hided. There is a parameter conflict between KVM and Hyper-V hypercall. > We hope L2 guest doesn't use KVM hypercall when the feature is > enabled. Detail please see comment of new API > "KVM_CAP_HYPERV_DIRECT_TLBFLUSH" I was thinking about this for awhile and I think I have a better proposal. Instead of adding this new capability let's enable direct TLB flush when KVM guest enables Hyper-V Hypercall page (writes to HV_X64_MSR_HYPERCALL) - this guarantees that the guest doesn't need KVM hypercalls as we can't handle both KVM-style and Hyper-V-style hypercalls simultaneously and kvm_emulate_hypercall() does: if (kvm_hv_hypercall_enabled(vcpu->kvm)) return kvm_hv_hypercall(vcpu); What do you think? (and instead of adding the capability we can add kvm.ko module parameter to enable direct tlb flush unconditionally, like 'hv_direct_tlbflush=-1/0/1' with '-1' being the default (autoselect based on Hyper-V hypercall enablement, '0' - permanently disabled, '1' - permanenetly enabled)). -- Vitaly