linux-kernel - Re: [PATCH v15 06/13] x86/sev: Prevent GUEST_TSC_FREQ MSR interception for Secure TSC enabled guests

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Message-ID: <7a5de2be-4e79-409a-90f2-398815fc59c7@amd.com>
Date: Wed, 18 Dec 2024 10:50:07 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Borislav Petkov <bp@...en8.de>
Cc: linux-kernel@...r.kernel.org, thomas.lendacky@....com, 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
Subject: Re: [PATCH v15 06/13] x86/sev: Prevent GUEST_TSC_FREQ MSR
 interception for Secure TSC enabled guests

On 12/17/2024 4:27 PM, Borislav Petkov wrote:
> On Thu, Dec 12, 2024 at 10:23:01AM +0530, Nikunj A. Dadhania wrote:
>> @@ -1477,19 +1480,13 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
>>  	case MSR_SVSM_CAA:
>>  		return __vc_handle_msr_caa(regs, write);
>>  	case MSR_IA32_TSC:
>> -		return __vc_handle_msr_tsc(regs, write);
>> +	case MSR_AMD64_GUEST_TSC_FREQ:
>> +		if (sev_status & MSR_AMD64_SNP_SECURE_TSC)
>> +			return __vc_handle_msr_tsc(regs, write);

With the above change, non-Secure TSC SNP guest will return following error:

$ sudo rdmsr 0xc0010134
rdmsr: CPU 0 cannot read MSR 0xc0010134
$

> Now push that conditional inside the function too.

With the condition inside the function, even tough the MSR is not valid in this configuration, I am getting value 0. Is this behavior acceptable ?

$ sudo rdmsr 0xc0010134
0
$

Regards,
Nikunj