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Message-ID: <c3ca63d6-db27-d783-40ca-486b3fbbced7@amd.com>
Date:   Thu, 21 Jul 2022 22:05:16 +0200
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Subject: Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM
 guest private memory
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To:     Sean Christopherson <seanjc@google.com>,
        Chao Peng <chao.p.peng@linux.intel.com>
Cc:     Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Price <steven.price@arm.com>,
        kvm list <kvm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        Linux API <linux-api@vger.kernel.org>, qemu-devel@nongnu.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        the arch/x86 maintainers <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Hugh Dickins <hughd@google.com>,
        Jeff Layton <jlayton@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
        "Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vishal Annapurve <vannapurve@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
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From:   "Gupta, Pankaj" <pankaj.gupta@amd.com>
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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?

Thanks,
Pankaj