Re: [PATCH v15 04/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests

[Tom Lendacky <thomas.lendacky@amd.com>]

On 12/9/24 23:02, Nikunj A. Dadhania wrote:
> On 12/9/2024 9:27 PM, Borislav Petkov wrote:
>> On Tue, Dec 03, 2024 at 02:30:36PM +0530, Nikunj A Dadhania wrote:
>>> Secure TSC enabled guests should not write to MSR_IA32_TSC(10H) register as
>>> the subsequent TSC value reads are undefined.
>>
>> What does that mean exactly?
> 
> That is the warning from the APM: 15.36.18 Secure TSC
> 
> "Guests that run with Secure TSC enabled are not expected to perform writes to
> the TSC MSR (10h). If such a write occurs, subsequent TSC values read are
> undefined."
> 
> What I make out of it is: if a write is performed to the TSC MSR, subsequent
> reads of TSC is not reliable/trusted.
> 
> That was the reason to ignore such writes in the #VC handler.
> 
>>
>> I'd prefer if we issued a WARN_ONCE() there on the write to catch any
>> offenders.
> 
> Do you also want to terminate the offending guest?
> 
> ES_UNSUPPORTED return will do that.
> 
>>
>> *NO ONE* should be writing the TSC MSR but that's a different story.
>>
>> IOW, something like this ontop of yours?
>>
>> ---
>>
>> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
>> index c22cb2ea4b99..050170eb28e6 100644
>> --- a/arch/x86/coco/sev/core.c
>> +++ b/arch/x86/coco/sev/core.c
>> @@ -1443,9 +1443,15 @@ static enum es_result __vc_handle_msr_tsc(struct pt_regs *regs, bool write)
>>  {
>>  	u64 tsc;
>>  
>> -	if (write)
>> -		return ES_OK;
>> +	if (!(sev_status & MSR_AMD64_SNP_SECURE_TSC))
>> +		goto read_tsc;
> 
> This is changing the behavior for SEV-ES and SNP guests(non SECURE_TSC), TSC MSR
> reads are converted to RDTSC. This is a good optimization. But just wanted to
> bring up the subtle impact.

Right, I think it should still flow through the GHCB MSR request for
non-Secure TSC guests.

> 
>> +
>> +	if (write) {
>> +		WARN_ONCE(1, "TSC MSR writes are verboten!\n");
>> +		return ES_UNSUPPORTED;
> 
> Sure, we can add a WARN_ONCE().

You'll want to test this... IIRC, I'm not sure if a WARN_ONCE() will be
properly printed when issued within the #VC handler (since it will
generate a nested #VC).

Thanks,
Tom

> 
>> +	}
>>  
>> +read_tsc:
>>  	tsc = rdtsc_ordered();
>>  	regs->ax = lower_32_bits(tsc);
>>  	regs->dx = upper_32_bits(tsc);
>> @@ -1462,11 +1468,14 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
>>  	/* Is it a WRMSR? */
>>  	write = ctxt->insn.opcode.bytes[1] == 0x30;
>>  
>> -	if (regs->cx == MSR_SVSM_CAA)
>> +	switch(regs->cx) {
> 
> Yes, I was thinking about a switch, as there will be more such instances when we
> enable newer features.
> 
>> +	case MSR_SVSM_CAA:
>>  		return __vc_handle_msr_caa(regs, write);
>> -
>> -	if (regs->cx == MSR_IA32_TSC && (sev_status & MSR_AMD64_SNP_SECURE_TSC))
>> +	case MSR_IA32_TSC:
>>  		return __vc_handle_msr_tsc(regs, write);
>> +	default:
>> +		break;
>> +	}
>>  
>>  	ghcb_set_rcx(ghcb, regs->cx);
>>  	if (write) {
>>
> 
> Regards,
> Nikunj