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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id k17sm8658535ejj.1.2020.11.30.08.59.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Nov 2020 08:59:09 -0800 (PST) Subject: Re: [PATCH 0/2] RFC: Precise TSC migration To: Andy Lutomirski , Maxim Levitsky Cc: kvm list , Oliver Upton , Ingo Molnar , Sean Christopherson , Thomas Gleixner , open list , Marcelo Tosatti , Jonathan Corbet , Wanpeng Li , Borislav Petkov , Jim Mattson , "H. Peter Anvin" , "open list:DOCUMENTATION" , Joerg Roedel , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Vitaly Kuznetsov References: <20201130133559.233242-1-mlevitsk@redhat.com> From: Paolo Bonzini Message-ID: <26444d65-083b-5df4-52c9-c1cfad556b10@redhat.com> Date: Mon, 30 Nov 2020 17:59:08 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 30/11/20 17:54, Andy Lutomirski wrote: >> I*do think* however that we should redefine KVM_CLOCK_TSC_STABLE >> in the documentation to state that it only guarantees invariance if the guest >> doesn't mess with its own TSC. >> >> Also I think we should consider enabling the X86_FEATURE_TSC_RELIABLE >> in the guest kernel, when kvm is detected to avoid the guest even from trying >> to sync TSC on newly hotplugged vCPUs. >> >> (The guest doesn't end up touching TSC_ADJUST usually, but it still might >> in some cases due to scheduling of guest vCPUs) >> >> (X86_FEATURE_TSC_RELIABLE short circuits tsc synchronization on CPU hotplug, >> and TSC clocksource watchdog, and the later we might want to keep). > If you're going to change the guest behavior to be more trusting of > the host, I think > the host should probably signal this to the guest using a new bit. > Yes, a new CPUID bit takes longer to propagate to existing setups, but it is more future proof. Paolo