Re: [PATCH v3 2/2] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall

[Paolo Bonzini]

On 04/05/21 22:33, Sean Christopherson wrote:
> On Tue, May 04, 2021, Paolo Bonzini wrote:
> > On 04/05/21 19:09, Sean Christopherson wrote:
> > > On Sat, May 01, 2021, Paolo Bonzini wrote:
> > > > - make it completely independent from migration, i.e. it's just a facet of
> > > > MSR_KVM_PAGE_ENC_STATUS saying whether the bitmap is up-to-date.  It would
> > > > use CPUID bit as the encryption status bitmap and have no code at all in KVM
> > > > (userspace needs to set up the filter and implement everything).
> > >
> > > If the bit is purely a "page encryption status is up-to-date", what about
> > > overloading KVM_HC_PAGE_ENC_STATUS to handle that status update as well?   That
> > > would eliminate my biggest complaint about having what is effectively a single
> > > paravirt feature split into two separate, but intertwined chunks of ABI.
> >
> > It's true that they are intertwined, but I dislike not having a way to read
> > the current state.
>
>  From the guest?

Yes, host userspace obviously doesn't need one since it's implemented 
through an MSR filter.  It may not be really necessary to read it, but 
it's a bit jarring compared to how the rest of the PV APIs uses MSRs.

Also from a debugging/crashdump point of view the VMM may have an 
established way to read an MSR from a vCPU, but it won't work if you 
come up with a new way to set the state.

Paolo