Messages in this thread | | | Subject | Re: Re: [PATCH v2 2/2] KVM: x86: use x86_get_freq to get freq for kvmclock | From | zhenwei pi <> | Date | Thu, 2 Dec 2021 13:26:27 +0800 |
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On 12/2/21 10:48 AM, Thomas Gleixner wrote: > On Wed, Dec 01 2021 at 10:46, zhenwei pi wrote: >> If the host side supports APERF&MPERF feature, the guest side may get >> mismatched frequency. >> >> KVM uses x86_get_cpufreq_khz() to get the same frequency for guest side. >> >> Signed-off-by: zhenwei pi <pizhenwei@bytedance.com> >> --- >> arch/x86/kvm/x86.c | 4 +--- >> 1 file changed, 1 insertion(+), 3 deletions(-) >> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index 5a403d92833f..125ed3c8b21a 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -8305,10 +8305,8 @@ static void tsc_khz_changed(void *data) >> >> if (data) >> khz = freq->new; >> - else if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) >> - khz = cpufreq_quick_get(raw_smp_processor_id()); >> if (!khz) >> - khz = tsc_khz; >> + khz = x86_get_cpufreq_khz(raw_smp_processor_id()); > > my brain compiler tells me that this is broken. > Without this patch: 1, boot_cpu_has(X86_FEATURE_CONSTANT_TSC) is true: no kvmclock_cpufreq_notifier, and khz = tsc_khz;
2, boot_cpu_has(X86_FEATURE_CONSTANT_TSC) is false: during installing kmod, try cpufreq_quick_get(), or use tsc_khz; and get changed by kvmclock_cpufreq_notifier.
With this patch: 1, boot_cpu_has(X86_FEATURE_CONSTANT_TSC) is true: no kvmclock_cpufreq_notifier, try aperf/mperf, or try cpufreq_quick_get(), or use cpu_khz
2, boot_cpu_has(X86_FEATURE_CONSTANT_TSC) is false: during installing kmod, try aperf/mperf, or try cpufreq_quick_get(), or use cpu_khz; and get changed by kvmclock_cpufreq_notifier.
I tested on Skylake&Icelake CPU, and got different CPU frequency from host & guest, the main purpose of this patch is to get the same frequency.
-- zhenwei pi
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