linux-kernel - Re: [PATCH v16 05/13] x86/sev: Add Secure TSC support for SNP guests

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Message-ID: <32359d64-357b-2104-59e8-4d3339a2197c@amd.com>
Date: Tue, 7 Jan 2025 12:53:26 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Borislav Petkov <bp@...en8.de>, "Nikunj A. Dadhania" <nikunj@....com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
 mingo@...hat.com, tglx@...utronix.de, dave.hansen@...ux.intel.com,
 pgonda@...gle.com, seanjc@...gle.com, pbonzini@...hat.com,
 francescolavra.fl@...il.com
Subject: Re: [PATCH v16 05/13] x86/sev: Add Secure TSC support for SNP guests

On 1/7/25 06:37, Borislav Petkov wrote:
> On Tue, Jan 07, 2025 at 05:13:03PM +0530, Nikunj A. Dadhania wrote:
>>>> +	case CC_ATTR_GUEST_SNP_SECURE_TSC:
>>>> +		return (sev_status & MSR_AMD64_SEV_SNP_ENABLED) &&
>>>
>>> This is new here?
>>
>> Yes, this was suggested by Tom here [1]
> 
> Either of you care to explain why this is needed?
> 
>>>> +			(sev_status & MSR_AMD64_SNP_SECURE_TSC);
> 
> I would strongly assume that whatever sets MSR_AMD64_SNP_SECURE_TSC will have
> checked/set MSR_AMD64_SEV_SNP_ENABLED already.

Yes, but from a readability point of view this makes it perfectly clear
that Secure TSC is only for SNP guests.

It's not on a fast path, so I don't think it hurts anything by having
the extra check. But if you prefer the previous check, I'm ok with that.

Thanks,
Tom

>