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

From: Steve Rutherford <srutherford@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>,
	LKML <linux-kernel@vger.kernel.org>,
	KVM list <kvm@vger.kernel.org>, Joerg Roedel <joro@8bytes.org>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Ashish Kalra <ashish.kalra@amd.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Borislav Petkov <bp@suse.de>, X86 ML <x86@kernel.org>
Subject: Re: [PATCH v3 2/2] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall
Date: Tue, 4 May 2021 14:16:20 -0700	[thread overview]
Message-ID: <CABayD+eAJjVjoh9GAnvC9z64pL9GnpHomCqXtw_=EPDr=vz7hA@mail.gmail.com> (raw)
In-Reply-To: <55db8e64-763b-9ecc-9c9a-6d840628e763@redhat.com>

On Tue, May 4, 2021 at 1:56 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> 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.

Agreed on the preference for an MSR. I particularly appreciate that it
reduces the kernel footprint for these changes.

Steve