On Fri, 2026-05-15 at 12:19 -0700, Sean Christopherson wrote: > Mark the TSC as reliable if the hypervisor (KVM) has enumerated the TSC > as constant and nonstop, and the admin hasn't explicitly marked the TSC > as unstable. Like most (all?) virtualization setups, any secondary > clocksource that's used as a watchdog is guaranteed to be less reliable > than a constant, nonstop TSC, as all clocksources the kernel uses as a > watchdog are all but guaranteed to be emulated when running as a KVM > guest. I.e. any observed discrepancies between the TSC and watchdog will > be due to jitter in the watchdog. > > This is especially true for KVM, as the watchdog clocksource is usually > emulated in host userspace, i.e. reading the clock incurs a roundtrip > cost of thousands of cycles. > > Marking the TSC reliable addresses a flaw where the TSC will occasionally > be marked unstable if the host is under moderate/heavy load. > > Signed-off-by: Sean Christopherson <[email protected]>
Reviewed-by: David Woodhouse <[email protected]>
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