linux-kernel - Re: [PATCH v12 14/19] tsc: Use the GUEST_TSC

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Message-ID: <9482e110-0837-d6e0-4f6c-a4b119da8c20@amd.com>
Date: Mon, 14 Oct 2024 09:06:58 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Tom Lendacky <thomas.lendacky@....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 v12 14/19] tsc: Use the GUEST_TSC_FREQ MSR for discovering
 TSC frequency

Hi Tom,

On 10/11/2024 1:09 AM, Tom Lendacky wrote:
> On 10/9/24 04:28, 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/msr-index.h |  1 +
>>  arch/x86/include/asm/sev.h       |  2 ++
>>  arch/x86/coco/sev/core.c         | 16 ++++++++++++++++
>>  arch/x86/kernel/tsc.c            |  5 +++++
>>  4 files changed, 24 insertions(+)

 
>> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
>> index 5f555f905fad..ef0def203b3f 100644
>> --- a/arch/x86/coco/sev/core.c
>> +++ b/arch/x86/coco/sev/core.c
>> @@ -3100,3 +3100,19 @@ void __init snp_secure_tsc_prepare(void)
>>  
>>  	pr_debug("SecureTSC enabled");
>>  }
>> +
>> +static unsigned long securetsc_get_tsc_khz(void)
>> +{
>> +	unsigned long long tsc_freq_mhz;
>> +
>> +	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
>> +	rdmsrl(MSR_AMD64_GUEST_TSC_FREQ, tsc_freq_mhz);
> 
> So this MSR can be intercepted by the hypervisor. You'll need to add
> code in the #VC handler that checks if an MSR access is for
> MSR_AMD64_GUEST_TSC_FREQ and Secure TSC is active, then the hypervisor
> is not cooperating and you should terminate the guest.

Yes, will add this in my next revision.

Regards
Nikunj