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([2001:b07:add:ec09:c399:bc87:7b6c:fb2a]) by smtp.gmail.com with ESMTPSA id cf4sm2709559edb.19.2021.03.15.08.45.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Mar 2021 08:45:33 -0700 (PDT) Subject: Re: [PATCH 2/4] KVM: x86: hyper-v: Prevent using not-yet-updated TSC page by secondary CPUs To: Vitaly Kuznetsov , kvm@vger.kernel.org Cc: Sean Christopherson , Wanpeng Li , Jim Mattson , Marcelo Tosatti References: <20210315143706.859293-1-vkuznets@redhat.com> <20210315143706.859293-3-vkuznets@redhat.com> From: Paolo Bonzini Message-ID: <6b392d7e-8135-53a9-9040-f6f5e316c6cb@redhat.com> Date: Mon, 15 Mar 2021 16:45:32 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <20210315143706.859293-3-vkuznets@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 15/03/21 15:37, Vitaly Kuznetsov wrote: > When KVM_REQ_MASTERCLOCK_UPDATE request is issued (e.g. after migration) > we need to make sure no vCPU sees stale values in PV clock structures and > thus all vCPUs are kicked with KVM_REQ_CLOCK_UPDATE. Hyper-V TSC page > clocksource is global and kvm_guest_time_update() only updates in on vCPU0 > but this is not entirely correct: nothing blocks some other vCPU from > entering the guest before we finish the update on CPU0 and it can read > stale values from the page. > > Call kvm_hv_setup_tsc_page() on all vCPUs. Normally, KVM_REQ_CLOCK_UPDATE > should be very rare so we may not care much about being wasteful. I think we should instead write 0 to the page in kvm_gen_update_masterclock. Paolo > Signed-off-by: Vitaly Kuznetsov > --- > arch/x86/kvm/x86.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 47e021bdcc94..882c509bfc86 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -2748,8 +2748,9 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) > offsetof(struct compat_vcpu_info, time)); > if (vcpu->xen.vcpu_time_info_set) > kvm_setup_pvclock_page(v, &vcpu->xen.vcpu_time_info_cache, 0); > - if (v == kvm_get_vcpu(v->kvm, 0)) > - kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock); > + > + kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock); > + > return 0; > } > >