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

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Date:   Mon, 25 Jul 2022 11:19:02 +0200
From:   "Gupta, Pankaj" <pankaj.gupta@....com>
To:     Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Chao Peng <chao.p.peng@...ux.intel.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        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


>>>> 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.
> 
> Even if we end up having a performance problem, I think we need to 
> understand the workloads that we want to optimize before getting too 
> excited about designing a speedup.
> 
> In particular, there's (depending on the specific technology, perhaps, 
> and also architecture) a possible tradeoff between trying to reduce 
> copying and trying to reduce unmapping and the associated flushes.  If a 
> user program maps an fd, populates it, and then converts it in place 
> into private memory (especially if it doesn't do it in a single shot), 
> then that memory needs to get unmapped both from the user mm and 
> probably from the kernel direct map.  On the flip side, it's possible to 
> imagine an ioctl that does copy-and-add-to-private-fd that uses a 
> private mm and doesn't need any TLB IPIs.
> 
> All of this is to say that trying to optimize right now seems quite 
> premature to me.

Agree to it. Thank you for explaining!

Thanks,
Pankaj