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

Nikunj A. Dadhania
2024-12-18T05:20:19Z

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