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Date:   Wed, 21 Jul 2021 20:15:16 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Brijesh Singh <brijesh.singh@amd.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-efi@vger.kernel.org, platform-driver-x86@vger.kernel.org,
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        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sergio Lopez <slp@redhat.com>, Peter Gonda <pgonda@google.com>,
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        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
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        Dov Murik <dovmurik@linux.ibm.com>,
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        Michael Roth <michael.roth@amd.com>,
        Vlastimil Babka <vbabka@suse.cz>, tony.luck@intel.com,
        npmccallum@redhat.com, brijesh.ksingh@gmail.com
Subject: Re: [PATCH Part2 RFC v4 37/40] KVM: SVM: Add support to handle the
 RMP nested page fault
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On Tue, Jul 20, 2021, Brijesh Singh wrote:
> 
> On 7/20/21 5:31 PM, Sean Christopherson wrote:
> ...
> >> This is a good question, the GHCB spec does not enforce that a guest *must*
> >> use page state. If the page state changes is not done by the guest then it
> >> will cause #NPF and its up to the hypervisor to decide on what it wants to
> >> do.
> > Drat.  Is there any hope of pushing through a GHCB change to require the guest
> > to use PSC?
> 
> Well, I am not sure if we can push it through GHCB. Other hypervisor
> also need to agree to it. We need to define them some architectural way
> for hypervisor to detect the violation and notify guest about it.

And other guest's, too :-/

> >>> It would simplify KVM (albeit not much of a simplificiation) and would also
> >>> make debugging easier since transitions would require an explicit guest
> >>> request and guest bugs would result in errors instead of random
> >>> corruption/weirdness.
> >> I am good with enforcing this from the KVM. But the question is, what fault
> >> we should inject in the guest when KVM detects that guest has issued the
> >> page state change.
> > Injecting a fault, at least from KVM, isn't an option since there's no architectural
> > behavior we can leverage.  E.g. a guest that isn't enlightened enough to properly
> > use PSC isn't going to do anything useful with a #MC or #VC.
> >
> > Sadly, as is I think our only options are to either automatically convert RMP
> > entries as need, or to punt the exit to userspace.  Maybe we could do both, e.g.
> > have a module param to control the behavior?  The problem with punting to userspace
> > is that KVM would also need a way for userspace to fix the issue, otherwise we're
> > just taking longer to kill the guest :-/
> >
> I think we should automatically convert the RMP entries at time, its
> possible that non Linux guest may access the page without going through
> the PSC.

Agreed.  I don't love that KVM will disallow automatic conversions when the host
is accessing guest memory, but not when the guest is accessing memory.  On the
other hand, auto-converting when accessing from the host is far, far worse.

And FWIW, IIRC this is also aligns with the expected/proposed TDX behavior, so
that's a plus.