linux-kernel - Re: [PATCH v3 2/2] KVM: X86: Introduce KVM_HC_PAGE_ENC

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Date:   Tue, 4 May 2021 17:09:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        srutherford@...gle.com, joro@...tes.org, brijesh.singh@....com,
        thomas.lendacky@....com, ashish.kalra@....com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
        x86@...nel.org
Subject: Re: [PATCH v3 2/2] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS
 hypercall

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;