From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH v1] kvm/x86: Hyper-V tsc page setup Date: Fri, 22 Jan 2016 12:02:08 +0100 Message-ID: <56A20C30.4020608@redhat.com> References: <1450949580-25759-1-git-send-email-asmetanin@virtuozzo.com> <56A1FF8F.3050701@redhat.com> <56A20131.8090102@virtuozzo.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: Gleb Natapov , "Denis V. Lunev" , Roman Kagan , qemu-devel@nongnu.org To: asmetanin@virtuozzo.com, kvm@vger.kernel.org Return-path: In-Reply-To: <56A20131.8090102@virtuozzo.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org Sender: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org List-Id: kvm.vger.kernel.org On 22/01/2016 11:15, Andrey Smetanin wrote: >> >> - unless KVM can use a master clock, it is incorrect to set up the TSC >> page this way; the sequence needs to be 0xFFFFFFFF in that case > 0xFFFFFFFF is not an invalid value for tsc page, > see https://lkml.org/lkml/2015/11/2/655 oh, I see now. >> - writing the TSC page must be done while all VCPUs are stopped, because >> the TSC page doesn't provide the possibility for the guest to retry in >> the middle of an update (like seqcount in Linux doess) > I think Windows guest gives tsc page address at boot time and protects > against other vcpu's tsc page access. Sometimes the TSC is detected to be unstable and Linux switches to another clocksource. At least in that case you can get a write to the TSC page while the guest is running. In that case it would be enough to write a zero to tsc_sequence, which _can_ be done atomically while the guest is running. However, KVM already has a mechanism to stop all VCPUs (KVM_REQ_MASTERCLOCK_UPDATE) so we might as well use it. Paolo