Re: [PATCH v3 33/41] x86/kvmclock: Mark TSC as reliable when it's constant and nonstop

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

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 <seanjc@xxxxxxxxxx>

Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

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