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([2001:b07:6468:f312:5e2c:eb9a:a8b6:fd3e]) by smtp.gmail.com with ESMTPSA id d14sm5110475edn.31.2020.12.10.03.42.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 10 Dec 2020 03:42:37 -0800 (PST) From: Paolo Bonzini Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE To: Thomas Gleixner , Vitaly Kuznetsov , Maxim Levitsky Cc: "H. Peter Anvin" , Jonathan Corbet , Jim Mattson , Wanpeng Li , "open list:KERNEL SELFTEST FRAMEWORK" , Marcelo Tosatti , Sean Christopherson , open list , Ingo Molnar , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Joerg Roedel , Borislav Petkov , Shuah Khan , Andrew Jones , Oliver Upton , "open list:DOCUMENTATION" , kvm@vger.kernel.org References: <20201203171118.372391-1-mlevitsk@redhat.com> <20201203171118.372391-2-mlevitsk@redhat.com> <87a6uq9abf.fsf@nanos.tec.linutronix.de> <1dbbeefc7c76c259b55582468ccd3aab35a6de60.camel@redhat.com> <87im9dlpsw.fsf@vitty.brq.redhat.com> <875z5d5x9m.fsf@nanos.tec.linutronix.de> Message-ID: Date: Thu, 10 Dec 2020 12:42:36 +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: <875z5d5x9m.fsf@nanos.tec.linutronix.de> 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 07/12/20 18:41, Thomas Gleixner wrote: > Right this happens still occasionally, but for quite some time this is > 100% firmware sillyness and not a fundamental property of the hardware > anymore. It's still a fundamental property of old hardware. Last time I tried to kill support for processors earlier than Core 2, I had to revert it. That's older than Nehalem. >> We try to catch such situation in KVM instead of blowing up but >> this may still result in subtle bugs I believe. Maybe we would be better >> off killing all VMs in case TSC ever gets unsynced (by default). > > I just ran a guest on an old machine with unsynchronized TSCs and was > able to observe clock monotonic going backwards between two threads > pinned on two vCPUs, which _is_ bad. Getting unsynced clocks reliably > under control is extremly hard. Using kvmclock? (Half serious: perhaps a good reason to have per-vCPU offsets is to be able to test what happens with unsynchronized TSCs...). Paolo