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([2001:b07:6468:f312:ec9b:111a:97e3:4baf]) by smtp.gmail.com with ESMTPSA id b187sm190321wmb.8.2020.09.25.13.32.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 25 Sep 2020 13:32:40 -0700 (PDT) Subject: Re: [PATCH v2 4/7] KVM: x86: hyper-v: always advertise HV_STIMER_DIRECT_MODE_AVAILABLE To: Vitaly Kuznetsov , kvm@vger.kernel.org Cc: Sean Christopherson , Wanpeng Li , Jim Mattson , Eduardo Habkost , Jon Doron , linux-kernel@vger.kernel.org References: <20200924145757.1035782-1-vkuznets@redhat.com> <20200924145757.1035782-5-vkuznets@redhat.com> From: Paolo Bonzini Message-ID: Date: Fri, 25 Sep 2020 22:32:40 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <20200924145757.1035782-5-vkuznets@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 24/09/20 16:57, Vitaly Kuznetsov wrote: > HV_STIMER_DIRECT_MODE_AVAILABLE is the last conditionally set feature bit > in KVM_GET_SUPPORTED_HV_CPUID but it doesn't have to be conditional: first, > this bit is only an indication to userspace VMM that direct mode stimers > are supported, it still requires manual enablement (enabling SynIC) to > work so no VMM should just blindly copy it to guest CPUIDs. Second, > lapic_in_kernel() is a must for SynIC. Expose the bit unconditionally. > > Signed-off-by: Vitaly Kuznetsov > --- > arch/x86/kvm/hyperv.c | 8 +------- > 1 file changed, 1 insertion(+), 7 deletions(-) > > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c > index 6da20f91cd59..503829f71270 100644 > --- a/arch/x86/kvm/hyperv.c > +++ b/arch/x86/kvm/hyperv.c > @@ -2028,13 +2028,7 @@ int kvm_vcpu_ioctl_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid, > ent->ebx |= HV_DEBUGGING; > ent->edx |= HV_X64_GUEST_DEBUGGING_AVAILABLE; > ent->edx |= HV_FEATURE_DEBUG_MSRS_AVAILABLE; > - > - /* > - * Direct Synthetic timers only make sense with in-kernel > - * LAPIC > - */ > - if (lapic_in_kernel(vcpu)) > - ent->edx |= HV_STIMER_DIRECT_MODE_AVAILABLE; > + ent->edx |= HV_STIMER_DIRECT_MODE_AVAILABLE; > > break; > > Sorry for the late reply. I think this is making things worse. It's obviously okay to add a system KVM_GET_SUPPORTED_HV_CPUID, and I guess it makes sense to have bits in there that require to enable a capability. For example, KVM_GET_SUPPORTED_CPUID has a couple bits such as X2APIC, that we return even if they require in-kernel irqchip. For the vCPU version however we should be able to copy the returned leaves to KVM_SET_CPUID2, meaning that unsupported features should be masked. Paolo