linux-kernel - Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM guest private memory

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Message-ID: <YtnCyqbI26QfRuOP@google.com>
Date:   Thu, 21 Jul 2022 21:19:06 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Gupta, Pankaj" <pankaj.gupta@....com>
Cc:     Chao Peng <chao.p.peng@...ux.intel.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Price <steven.price@....com>,
        kvm list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>, qemu-devel@...gnu.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        nikunj@....com, ashish.kalra@....com
Subject: Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM
 guest private memory

On Thu, Jul 21, 2022, Gupta, Pankaj wrote:
> 
> Hi Sean, Chao,
> 
> While attempting to solve the pre-boot guest payload/firmware population
> into private memory for SEV SNP, retrieved this thread. Have question below:
> 
> > > > Requirements & Gaps
> > > > -------------------------------------
> > > >    - Confidential computing(CC): TDX/SEV/CCA
> > > >      * Need support both explicit/implicit conversions.
> > > >      * Need support only destructive conversion at runtime.
> > > >      * The current patch should just work, but prefer to have pre-boot guest
> > > >        payload/firmware population into private memory for performance.
> > > 
> > > Not just performance in the case of SEV, it's needed there because firmware
> > > only supports in-place encryption of guest memory, there's no mechanism to
> > > provide a separate buffer to load into guest memory at pre-boot time. I
> > > think you're aware of this but wanted to point that out just in case.
> > 
> > I view it as a performance problem because nothing stops KVM from copying from
> > userspace into the private fd during the SEV ioctl().  What's missing is the
> > ability for userspace to directly initialze the private fd, which may or may not
> > avoid an extra memcpy() depending on how clever userspace is.
> Can you please elaborate more what you see as a performance problem? And
> possible ways to solve it?

Oh, I'm not saying there actually _is_ a performance problem.  What I'm saying is
that in-place encryption is not a functional requirement, which means it's purely
an optimization, and thus we should other bother supporting in-place encryption
_if_ it would solve a performane bottleneck.