Ping? On Fri, Jan 04, 2019 at 03:54:14PM -0200, Marcelo Tosatti wrote: > > > The invariant TSC bit has the following meaning: > > "The time stamp counter in newer processors may support an enhancement, > referred to as invariant TSC. Processor's support for invariant TSC > is indicated by CPUID.80000007H:EDX[8]. The invariant TSC will run > at a constant rate in all ACPI P-, C-. and T-states. This is the > architectural behavior moving forward. On processors with invariant TSC > support, the OS may use the TSC for wall clock timer services (instead > of ACPI or HPET timers). TSC reads are much more efficient and do not > incur the overhead associated with a ring transition or access to a > platform resource." > > IOW, TSC does not change frequency. In such case, and with > TSC scaling hardware available to handle migration, it is possible > to use the TSC clocksource directly, whose system calls are > faster. > > Reduce the rating of kvmclock clocksource to allow TSC clocksource > to be the default if invariant TSC is exposed. > > Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> > > v2: Use feature bits and tsc_unstable() check (Sean Christopherson) > > diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c > index 30084ec..a14601c 100644 > --- a/arch/x86/kernel/kvmclock.c > +++ b/arch/x86/kernel/kvmclock.c > @@ -368,6 +368,20 @@ void __init kvmclock_init(void) > machine_ops.crash_shutdown = kvm_crash_shutdown; > #endif > kvm_get_preset_lpj(); > + > + /* > + * X86_FEATURE_NONSTOP_TSC is TSC runs at constant rate > + * with P/T states and does not stop in deep C-states. > + * > + * Invariant TSC exposed by host means kvmclock is not necessary: > + * can use TSC as clocksource. > + * > + */ > + if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && > + boot_cpu_has(X86_FEATURE_NONSTOP_TSC) && > + !check_tsc_unstable()) > + kvm_clock.rating = 299; > + > clocksource_register_hz(&kvm_clock, NSEC_PER_SEC); > pv_info.name = "KVM"; > } >
