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

[Sean Christopherson <seanjc@google.com>]

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.

#define KVM_HC_PAGE_ENC_UPDATE		12

#define KVM_HC_PAGE_ENC_REGION_UPDATE	0 /* encrypted vs. plain text */
#define KVM_HC_PAGE_ENC_STATUS_UPDATE	1 /* up-to-date vs. stale */

		ret = -KVM_ENOSYS;
		if (!vcpu->kvm->arch.hypercall_exit_enabled)
		        break;

		ret = -EINVAL;
		if (a0 == KVM_HC_PAGE_ENC_REGION_UPDATE) {
			u64 gpa = a1, npages = a2;

			if (!PAGE_ALIGNED(gpa) || !npages ||
			    gpa_to_gfn(gpa) + npages <= gpa_to_gfn(gpa))
				break;
		} else if (a0 != KVM_HC_PAGE_ENC_STATUS_UPDATE) {
			break;
		}

		vcpu->run->exit_reason        = KVM_EXIT_HYPERCALL;
		vcpu->run->hypercall.nr       = KVM_HC_PAGE_ENC_STATUS;
		vcpu->run->hypercall.args[0]  = a0;
		vcpu->run->hypercall.args[1]  = a1;
		vcpu->run->hypercall.args[2]  = a2;
		vcpu->run->hypercall.args[3]  = a3;
		vcpu->run->hypercall.longmode = op_64_bit;
		vcpu->arch.complete_userspace_io = complete_hypercall_exit;
		return 0;