Date:   Sat, 13 Nov 2021 18:34:52 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Marc Orr <marcorr@google.com>
Cc:     Peter Gonda <pgonda@google.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Brijesh Singh <brijesh.singh@amd.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-mm@kvack.org,
        linux-crypto@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        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 Zijlstra <peterz@infradead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dov Murik <dovmurik@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Michael Roth <Michael.Roth@amd.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Andi Kleen <ak@linux.intel.com>, tony.luck@intel.com,
        sathyanarayanan.kuppuswamy@linux.intel.com
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support
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On Fri, Nov 12, 2021, Marc Orr wrote:
> > > > If *it* is the host kernel, then you probably shouldn't do that -
> > > > otherwise you just killed the host kernel on which all those guests are
> > > > running.
> > >
> > > I agree, it seems better to terminate the single guest with an issue.
> > > Rather than killing the host (and therefore all guests). So I'd
> > > suggest even in this case we do the 'convert to shared' approach or
> > > just outright terminate the guest.
> > >
> > > Are there already examples in KVM of a KVM bug in servicing a VM's
> > > request results in a BUG/panic/oops? That seems not ideal ever.
> >
> > Plenty of examples.  kvm_spurious_fault() is the obvious one.  Any NULL pointer
> > deref will lead to a BUG, etc...  And it's not just KVM, e.g. it's possible, if
> > unlikely, for the core kernel to run into guest private memory (e.g. if the kernel
> > botches an RMP change), and if that happens there's no guarantee that the kernel
> > can recover.
> >
> > I fully agree that ideally KVM would have a better sense of self-preservation,
> > but IMO that's an orthogonal discussion.
> 
> I don't think we should treat the possibility of crashing the host
> with live VMs nonchalantly. It's a big deal. Doing so has big
> implications on the probability that any cloud vendor wil bee able to
> deploy this code to production. And aren't cloud vendors one of the
> main use cases for all of this confidential compute stuff? I'm
> honestly surprised that so many people are OK with crashing the host.

I'm not treating it nonchalantly, merely acknowledging that (a) some flavors of kernel
bugs (or hardware issues!) are inherently fatal to the system, and (b) crashing the
host may be preferable to continuing on in certain cases, e.g. if continuing on has a
high probablity of corrupting guest data.