Re: [PATCH v15 09/13] tsc: Use the GUEST_TSC_FREQ MSR for discovering TSC frequency

Tom Lendacky
2024-12-17T07:05:45Z

On 12/17/24 00:27, Nikunj A Dadhania wrote:
> On 12/16/2024 10:01 PM, Tom Lendacky wrote:
>> On 12/3/24 03:00, Nikunj A Dadhania wrote:
>>> Calibrating the TSC frequency using the kvmclock is not correct for
>>> SecureTSC enabled guests. Use the platform provided TSC frequency via the
>>> GUEST_TSC_FREQ MSR (C001_0134h).
>>>
>>> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
>>> ---
>>> arch/x86/include/asm/sev.h | 2 ++
>>> arch/x86/coco/sev/core.c | 16 ++++++++++++++++
>>> arch/x86/kernel/tsc.c | 5 +++++
>>> 3 files changed, 23 insertions(+)
>>>

> @@ -3282,16 +3283,18 @@ void __init snp_secure_tsc_prepare(void)
>
> static unsigned long securetsc_get_tsc_khz(void)
> {
> - unsigned long long tsc_freq_mhz;
> -
> setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);

I was thinking even this can be moved.

Thanks,
Tom

> - rdmsrl(MSR_AMD64_GUEST_TSC_FREQ, tsc_freq_mhz);
>
> - return (unsigned long)(tsc_freq_mhz * 1000);
> + return snp_tsc_freq_khz;
> }
>
> void __init snp_secure_tsc_init(void)
> {
> + unsigned long long tsc_freq_mhz;
> +
> + rdmsrl(MSR_AMD64_GUEST_TSC_FREQ, tsc_freq_mhz);
> + snp_tsc_freq_khz = (unsigned long)(tsc_freq_mhz * 1000);
> +
> x86_platform.calibrate_cpu = securetsc_get_tsc_khz;
> x86_platform.calibrate_tsc = securetsc_get_tsc_khz;
> }
>
>
> ---
>
> Regards
> Nikunj