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[91.219.240.17]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48897b8176fsm183373255e9.0.2026.04.07.01.17.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Apr 2026 01:17:01 -0700 (PDT) From: Vitaly Kuznetsov To: Thomas Lefebvre , seanjc@google.com, pbonzini@redhat.com Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org Subject: Re: [BUG] KVM: x86: kvmclock jumps ~253 years on Hyper-V nested virt due to cross-CPU raw TSC inconsistency In-Reply-To: References: Date: Tue, 07 Apr 2026 10:17:00 +0200 Message-ID: <87v7e3mbgj.fsf@redhat.com> Precedence: bulk X-Mailing-List: linux-hyperv@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Thomas Lefebvre writes: ... > > Under Hyper-V, raw RDTSC values are not consistent across vCPUs. > The hypervisor corrects them only through the TSC page scale/offset. > If pvclock_update_vm_gtod_copy() runs on CPU 0 and __get_kvmclock() > later runs on CPU 1 where the raw TSC is lower, the unsigned > subtraction wraps. > According to the TLFS, reference TSC page is partition wide: "The hypervisor provides a partition-wide virtual reference TSC page which is overlaid on the partition=E2=80=99s GPA space. A partition=E2=80= =99s reference time stamp counter page is accessed through the Reference TSC MSR." so if as you say RAW rdtsc value is inconsistent across vCPUs, I can hardly see how we can use this time source at all, even without KVM. scale/offset are the same for all vCPUs. I think the fix here is to avoid setting up Hyper-V TSC page clocksource in L1. Unfortunately, with unsynchronized TSCs this will leave us the only choice for a sane clocksource: raw HV_X64_MSR_TIME_REF_COUNT MSR reads. --=20 Vitaly