Re: [PATCH v13 08/12] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall

[Sean Christopherson]

On Tue, Apr 20, 2021, Paolo Bonzini wrote:
> On 15/04/21 17:57, Ashish Kalra wrote:
> > From: Ashish Kalra <ashish.kalra@xxxxxxx>
> > 
> > This hypercall is used by the SEV guest to notify a change in the page
> > encryption status to the hypervisor. The hypercall should be invoked
> > only when the encryption attribute is changed from encrypted -> decrypted
> > and vice versa. By default all guest pages are considered encrypted.
> > 
> > The hypercall exits to userspace to manage the guest shared regions and
> > integrate with the userspace VMM's migration code.
> 
> I think this should be exposed to userspace as a capability, rather than as
> a CPUID bit.  Userspace then can enable the capability and set the CPUID bit
> if it wants.
> 
> The reason is that userspace could pass KVM_GET_SUPPORTED_CPUID to
> KVM_SET_CPUID2 and the hypercall then would break the guest.

Right, and that's partly why I was advocating that KVM emulate the MSR as a nop.