kvmclock: fix TSC calibration for nested guests
Inside a nested guest, access to hardware can be slow enough that tsc_read_refs always return ULLONG_MAX, causing tsc_refine_calibration_work to be called periodically and the nested guest to spend a lot of time reading the ACPI timer. However, if the TSC frequency is available from the pvclock page, we can just set X86_FEATURE_TSC_KNOWN_FREQ and avoid the recalibration. 'refine' operation. Suggested-by: NPeter Zijlstra <peterz@infradead.org> Signed-off-by: NPeng Hao <peng.hao2@zte.com.cn> [Commit message rewritten. - Paolo] Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
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