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Date:   Wed, 30 Mar 2022 19:47:33 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Nikunj A. Dadhania" <nikunj@amd.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Peter Gonda <pgonda@google.com>,
        Bharata B Rao <bharata@amd.com>,
        "Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
        Mingwei Zhang <mizhang@google.com>,
        David Hildenbrand <david@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC v1 0/9] KVM: SVM: Defer page pinning for SEV guests
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References: <20220308043857.13652-1-nikunj@amd.com>
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On Wed, Mar 30, 2022, Nikunj A. Dadhania wrote:
> On 3/29/2022 2:30 AM, Sean Christopherson wrote:
> > Let me preface this by saying I generally like the idea and especially the
> > performance, but...
> > 
> > I think we should abandon this approach in favor of committing all our resources
> > to fd-based private memory[*], which (if done right) will provide on-demand pinning
> > for "free".  
> 
> I will give this a try for SEV, was on my todo list.
> 
> > I would much rather get that support merged sooner than later, and use
> > it as a carrot for legacy SEV to get users to move over to its new APIs, with a long
> > term goal of deprecating and disallowing SEV/SEV-ES guests without fd-based private
> > memory.  
> 
> > That would require guest kernel support to communicate private vs. shared,
> 
> Could you explain this in more detail? This is required for punching hole for shared pages?

Unlike SEV-SNP, which enumerates private vs. shared in the error code, SEV and SEV-ES
don't provide private vs. shared information to the host (KVM) on page fault.  And
it's even more fundamental then that, as SEV/SEV-ES won't even fault if the guest
accesses the "wrong" GPA variant, they'll silent consume/corrupt data.

That means KVM can't support implicit conversions for SEV/SEV-ES, and so an explicit
hypercall is mandatory.  SEV doesn't even have a vendor-agnostic guest/host paravirt
ABI, and IIRC SEV-ES doesn't provide a conversion/map hypercall in the GHCB spec, so
running a SEV/SEV-ES guest under UPM would require the guest firmware+kernel to be
properly enlightened beyond what is required architecturally.