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

[Ashish Kalra]

On Wed, Feb 19, 2020 at 06:39:39PM -0800, Steve Rutherford wrote:
> On Wed, Feb 12, 2020 at 5:17 PM Ashish Kalra <Ashish.Kalra@xxxxxxx> wrote:
> >
> >  static void vmx_cleanup_l1d_flush(void)
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index fbabb2f06273..298627fa3d39 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -7547,6 +7547,12 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> >                 kvm_sched_yield(vcpu->kvm, a0);
> >                 ret = 0;
> >                 break;
> > +       case KVM_HC_PAGE_ENC_STATUS:
> > +               ret = -KVM_ENOSYS;
> > +               if (kvm_x86_ops->page_enc_status_hc)
> > +                       ret = kvm_x86_ops->page_enc_status_hc(vcpu->kvm,
> > +                                       a0, a1, a2);
> > +               break;
> >         default:
> >                 ret = -KVM_ENOSYS;
> >                 break;
> Add a cap to kvm_vm_ioctl_enable_cap so that the vmm can configure
> whether or not this hypercall is offered. Moving to an enable cap
> would also allow the vmm to pass down the expected size of the c-bit
> tracking buffer, so that you don't need to handle dynamic resizing in
> response to guest hypercall, otherwise KVM will sporadically start
> copying around large buffers when working with large VMs.
> 

Yes, that is something we have been looking at adding.

But, how will VMM know the expected size of the c-bit tracking buffer ?

The guest kernel and firmware make the hypercall to mark page encryption
status and depending on the GPA range being marked, the kernel's page
encryption bitmap needs to be dynamically resized as response to the guest
hypercall.

> Stepping back a bit, I'm a little surprised by the fact that you don't
> treat the c-bit buffers the same way as the dirty tracking buffers and
> put them alongside the memslots. That's probably more effort, and the
> strategy of using one large buffer should work fine (assuming you
> don't need to support non-zero as_ids).

Thanks,
Ashish