linux-kernel - Re: [PATCH RFC v1 0/9] KVM: SVM: Defer page pinning for SEV guests

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Message-ID: <CAMkAt6px4A0CyuZ8h7zKzTxQUrZMYEkDXbvZ=3v+kphRTRDjNA@mail.gmail.com>
Date:   Thu, 31 Mar 2022 12:32:18 -0600
From:   Peter Gonda <pgonda@...gle.com>
To:     "Nikunj A. Dadhania" <nikunj@....com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Bharata B Rao <bharata@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Mingwei Zhang <mizhang@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v1 0/9] KVM: SVM: Defer page pinning for SEV guests

On Wed, Mar 30, 2022 at 10:48 PM Nikunj A. Dadhania <nikunj@....com> wrote:
>
>
>
> On 3/31/2022 1:17 AM, Sean Christopherson wrote:
> > 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.
> >
>
> So with guest supporting KVM_FEATURE_HC_MAP_GPA_RANGE and host (KVM) supporting
> KVM_HC_MAP_GPA_RANGE hypercall, SEV/SEV-ES guest should communicate private/shared
> pages to the hypervisor, this information can be used to mark page shared/private.

One concern here may be that the VMM doesn't know which guests have
KVM_FEATURE_HC_MAP_GPA_RANGE support and which don't. Only once the
guest boots does the guest tell KVM that it supports
KVM_FEATURE_HC_MAP_GPA_RANGE. If the guest doesn't we need to pin all
the memory before we run the guest to be safe to be safe.