linux-kernel - Re: [PATCH v15 09/13] tsc: Use the GUEST_TSC

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Message-ID: <1c7734a4-e6f2-378b-cef2-af087a51526b@amd.com>
Date: Tue, 17 Dec 2024 01:05:41 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Nikunj A Dadhania <nikunj@....com>, linux-kernel@...r.kernel.org,
 bp@...en8.de, x86@...nel.org, kvm@...r.kernel.org
Cc: mingo@...hat.com, tglx@...utronix.de, dave.hansen@...ux.intel.com,
 pgonda@...gle.com, seanjc@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v15 09/13] tsc: Use the GUEST_TSC_FREQ MSR for discovering
 TSC frequency

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@....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