Re: [PATCH kernel v3 3/3] x86/sev: Do not handle #VC for DR7 read/write

[Nikunj A. Dadhania]

On 24/01/23 18:07, Alexey Kardashevskiy wrote:
> 
> 
> On 24/1/23 21:37, Nikunj A. Dadhania wrote:
>> It is MSR_AMD64_SEV_DEBUG_SWAP (SEV, not SNP), it is an SEV-ES thing.
> Yes, noticed that, earlier analysis was that Debug Swap shouldn't need any guest side changes, but it does need it.

>>> Why is that feature negotiation SNP-only and not SEV?

>> As per the spec, GHCB termination request: reason code: 0x2 is SNP features specific.
> Does the guest really need to terminate in such case? 

The termination is from the guest that do not have implementation for the hypervisor enabled feature, in this case DebugSwap. 
If DebugSwap is enabled by the hypervisor and not handled in guest #VC, then DR7 read/write can be intercepted by the malicious
hypervisor, which can return unexpected values.

> A VM could just not do the GHCB thing if it does not want to.

In that case, the VM can have unexpected failures.

Regards
Nikunj