Re: [PATCH v13 04/13] x86/sev: Change TSC MSR behavior for Secure TSC enabled guests

Nikunj A. Dadhania
2024-10-24T10:17:07Z

On 10/24/2024 1:01 PM, Xiaoyao Li wrote:
> On 10/24/2024 2:24 PM, Nikunj A. Dadhania wrote:
>>
>>
>> On 10/23/2024 8:55 AM, Xiaoyao Li wrote:
>>> On 10/21/2024 1:51 PM, Nikunj A Dadhania wrote:
>>>> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
>>>> index 965209067f03..2ad7773458c0 100644
>>>> --- a/arch/x86/coco/sev/core.c
>>>> +++ b/arch/x86/coco/sev/core.c
>>>> @@ -1308,6 +1308,30 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
>>>> return ES_OK;
>>>> }
>>>> + /*
>>>> + * TSC related accesses should not exit to the hypervisor when a
>>>> + * guest is executing with SecureTSC enabled, so special handling
>>>> + * is required for accesses of MSR_IA32_TSC:
>>>> + *
>>>> + * Writes: Writing to MSR_IA32_TSC can cause subsequent reads
>>>> + * of the TSC to return undefined values, so ignore all
>>>> + * writes.
>>>> + * Reads: Reads of MSR_IA32_TSC should return the current TSC
>>>> + * value, use the value returned by RDTSC.
>>>> + */
>>>
>>>
>>> Why doesn't handle it by returning ES_VMM_ERROR when hypervisor
>>> intercepts RD/WR of MSR_IA32_TSC? With SECURE_TSC enabled, it seems
>>> not need to be intercepted.
>>
>> ES_VMM_ERROR will terminate the guest, which is not the expected
>> behaviour. As documented, writes to the MSR is ignored and reads are
>> done using RDTSC.
>>
>>> I think the reason is that SNP guest relies on interception to do
>>> the ignore behavior for WRMSR in #VC handler because the writing
>>> leads to undefined result.
>>
>> For legacy and secure guests MSR_IA32_TSC is always intercepted(for
>> both RD/WR).
>
> We cannot make such assumption unless it's enforced by AMD HW.
>
>> Moreover, this is a legacy MSR, RDTSC and RDTSCP is the what modern
>> OSes should use.
>
> Again, this is your assumption and expectation only.

Not my assumption, I could not find any reference to MSR_IA32_TSC
read/write in the kernel code.

>
>> The idea is the catch any writes to TSC MSR and handle them
>> gracefully.
>
> If SNP guest requires MSR_IA32_TSC being intercepted by hypervisor. It
> should come with a solution that guest kernel can check it certainly,
> just like the patch 5 and patch 6, that they can check the behavior of
> hypervisor.
>
> If there is no clean way for guest to ensure MSR_IA32_TSC is
> intercepted by hypervisor,

Yes, that is my understanding as well.

> we at least need add some comment to call
> out that these code replies on the assumption that hypervisor
> intercepts MSR_IA32_TSC.

Sure, I will add a comment.

>>> Then the question is what if the hypervisor doesn't intercept write
>>> to MSR_IA32_TSC in the first place?
>>
>> I have tried to disable interception of MSR_IA32_TSC, and writes are
>> ignored by the HW as well. I would like to continue the current
>> documented HW as per the APM.
>
> I only means the writes are ignored in your testing HW. We don't know
> the result on other SNP-capable HW or future HW, unless it's
> documented in APM.
>
> Current documented behavior is that write leads to a undefined value
> of subsequent read. So we need to avoid write. One solution is to
> intercept write and ignore it, but it depends on hypervisor to
> intercept it.
> Anther solution would be we fix all the place of writing
> MSR_IA32_TSC for SNP guest in linux.

There is no MSR_IA32_TSC write in the kernel code, IMHO, so second
solution is taken care.

Regards,
Nikunj