Re: [kvm-unit-tests 02/13] x86: AMD SEV-ES: Setup #VC exception handler for AMD SEV-ES

From: Sean Christopherson <seanjc@google.com>
To: Marc Orr <marcorr@google.com>
Cc: Joerg Roedel <jroedel@suse.de>,
	Varad Gautam <varad.gautam@suse.com>,
	kvm list <kvm@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Andrew Jones <drjones@redhat.com>,
	Zixuan Wang <zxwang42@gmail.com>,
	Erdem Aktas <erdemaktas@google.com>,
	David Rientjes <rientjes@google.com>,
	"Singh, Brijesh" <brijesh.singh@amd.com>,
	"Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
	bp@suse.de
Subject: Re: [kvm-unit-tests 02/13] x86: AMD SEV-ES: Setup #VC exception handler for AMD SEV-ES
Date: Fri, 4 Feb 2022 16:30:34 +0000	[thread overview]
Message-ID: <Yf1UqmkfirgX1Nl+@google.com> (raw)
In-Reply-To: <CAA03e5HnyqZqDOyK8cbJgq_-zMPYEcrAuKr_CF8+=3DeykfV5A@mail.gmail.com>

On Fri, Feb 04, 2022, Marc Orr wrote:
> On Fri, Feb 4, 2022 at 2:55 AM Joerg Roedel <jroedel@suse.de> wrote:
> >         3) The firmware #VC handler might use state which is not
> >            available anymore after ExitBootServices.
> 
> Of all the issues listed, this one seems the most serious.
> 
> >         4) If the firmware uses the kvm-unit-test GHCB after
> >            ExitBootServices, it has the get the GHCB address from the
> >            GHCB MSR, requiring an identity mapping.
> >            Moreover it requires to keep the address of the GHCB in the
> >            MSR at all times where a #VC could happen. This could be a
> >            problem when we start to add SEV-ES specific tests to the
> >            unit-tests, explcitily testing the MSR protocol.
> 
> Ack. I'd think we could require tests to save/restore the GHCB MSR.
> 
> > It is easy to violate this implicit protocol and breaking kvm-unit-tests
> > just by a new version of OVMF being used. I think that is not a very
> > robust approach and a separate #VC handler in the unit-test framework
> > makes sense even now.
> 
> Thanks for the explanation! I hope we can keep the UEFI #VC handler
> working, because like I mentioned, I think this work can be used to
> test that code inside of UEFI. But I guess time will tell.
> 
> Of all the points listed above, I think point #3 is the most
> concerning. The others seem like they can be managed.

  5) Debug.  I don't want to have to reverse engineer assembly code to understand
     why a #VC handler isn't doing what I expect, or to a debug the exchanges
     between guest and host.

On Thu, Jan 20, 2022 at 4:52 AM Varad Gautam <varad.gautam@suse.com> wrote:
> If --amdsev-efi-vc is passed during ./configure, the tests will
> continue using the UEFI #VC handler.

Why bother?  I would prefer we ditch the UEFI #VC handler entirely and not give
users the option to using anything but the built-in handler.  What do we gain
other than complexity?