On Thu, Jan 03, 2019 at 05:18:49PM -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>
>
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index 30084ec..575857c 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -35,6 +35,7 @@
> #include <asm/x86_init.h>
> #include <asm/reboot.h>
> #include <asm/kvmclock.h>
> +#include <asm/processor.h>
>
> static int kvmclock __initdata = 1;
> static int kvmclock_vsyscall __initdata = 1;
> @@ -325,6 +326,7 @@ static int kvmclock_setup_percpu(unsigned int cpu)
> void __init kvmclock_init(void)
> {
> u8 flags;
> + struct cpuinfo_x86 *c = &boot_cpu_data;
>
> if (!kvm_para_available() || !kvmclock)
> return;
> @@ -368,6 +370,18 @@ void __init kvmclock_init(void)
> machine_ops.crash_shutdown = kvm_crash_shutdown;
> #endif
> kvm_get_preset_lpj();
> +
> + /*
> + * c->x86_power is 8000_0007 edx. Bit 8 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 (c->x86_power & (1 << 8))
> + kvm_clock.rating = 299;
Why check x86_power manually instead of using the feature bits that all
x86 CPUs set based on x86_power? And it seems like this should check
tsc_unstable as well, e.g. to account for the user explicitly disabling
the TSC via kernel params.
I.e.:
if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
!check_tsc_unstable())
kvm_clock.rating = 299;
Note that I have no idea if there are other changes necessary to promote
the TSC above kvmclock, i.e. someone with more clock knowledge should
review this :)
> +
> clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);
> pv_info.name = "KVM";
> }
>
>
>
>
