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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id 61sm15690877wrm.52.2021.05.24.23.23.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 24 May 2021 23:23:20 -0700 (PDT) From: Vitaly Kuznetsov To: Paolo Bonzini , kvm@vger.kernel.org Cc: Sean Christopherson , Wanpeng Li , Jim Mattson , Maxim Levitsky , Kechen Lu , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 5/5] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use In-Reply-To: <82e2a6a0-337a-4b92-2271-493344a38960@redhat.com> References: <20210518144339.1987982-1-vkuznets@redhat.com> <20210518144339.1987982-6-vkuznets@redhat.com> <82e2a6a0-337a-4b92-2271-493344a38960@redhat.com> Date: Tue, 25 May 2021 08:23:19 +0200 Message-ID: <87fsyb8h7s.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Paolo Bonzini writes: > On 18/05/21 16:43, Vitaly Kuznetsov wrote: >> APICV_INHIBIT_REASON_HYPERV is currently unconditionally forced upon >> SynIC activation as SynIC's AutoEOI is incompatible with APICv/AVIC. It is, >> however, possible to track whether the feature was actually used by the >> guest and only inhibit APICv/AVIC when needed. >> >> TLFS suggests a dedicated 'HV_DEPRECATING_AEOI_RECOMMENDED' flag to let >> Windows know that AutoEOI feature should be avoided. While it's up to >> KVM userspace to set the flag, KVM can help a bit by exposing global >> APICv/AVIC enablement: in case APICv/AVIC usage is impossible, AutoEOI >> is still preferred. >> >> Signed-off-by: Vitaly Kuznetsov > > Should it also disable APICv unconditionally if > HV_DEPRECATING_AEOI_RECOMMENDED is not in the guest CPUID? That should > avoid ping-pong between enabled and disabled APICv even in pathological > cases that we cannot think about. When you run Hyper-V on KVM it doesn't use SynIC (let alone AutoEOI) but we still inhibit APICv unconditionally. The patch as-is improves this without any userspace changes required and I see it as a benefit. Going forward, we will definitely add something like 'hv-synic-noaeoi' to QEMU to make non-nesting setups benefit too but it'll take a while for this option to propagate to real world configurations (sigh). -- Vitaly