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[54.240.197.228]) by smtp.gmail.com with ESMTPSA id n15-20020a05600c500f00b00417c0fa4b82sm729506wmr.25.2024.04.10.05.43.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Apr 2024 05:43:27 -0700 (PDT) From: Paul Durrant X-Google-Original-From: Paul Durrant Message-ID: <26bfe5ec-e583-458d-8e43-e5ecdc5883cc@xen.org> Date: Wed, 10 Apr 2024 13:43:25 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Reply-To: paul@xen.org Subject: Re: [PATCH v2 1/2] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate KVM clock migration To: David Woodhouse , Paul Durrant , Jack Allister Cc: bp@alien8.de, corbet@lwn.net, dave.hansen@linux.intel.com, hpa@zytor.com, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@redhat.com, pbonzini@redhat.com, seanjc@google.com, tglx@linutronix.de, x86@kernel.org, Dongli Zhang References: <20240408220705.7637-1-jalliste@amazon.com> <20240410095244.77109-1-jalliste@amazon.com> <20240410095244.77109-2-jalliste@amazon.com> <005911c5-7f9d-4397-8145-a1ad4494484d@xen.org> Content-Language: en-US Organization: Xen Project In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10/04/2024 13:09, David Woodhouse wrote: > On 10 April 2024 11:29:13 BST, Paul Durrant wrote: >> On 10/04/2024 10:52, Jack Allister wrote: >>> + * It's possible that this vCPU doesn't have a HVCLOCK configured >>> + * but the other vCPUs may. If this is the case calculate based >>> + * upon the time gathered in the seqcount but do not update the >>> + * vCPU specific PVTI. If we have one, then use that. >> >> Given this is a per-vCPU ioctl, why not fail in the case the vCPU doesn't have HVCLOCK configured? Or is your intention that a GET/SET should always work if TSC is stable? > > It definitely needs to work for SET even when the vCPU hasn't been run yet (and doesn't have a hvclock in vcpu->arch.hv_clock). So would it make sense to set up hvclock earlier?