[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <17ce3189-5e88-3c9b-605d-e259dcedece3@amd.com>
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
Powered by blists - more mailing lists