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SubjectRe: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM guest private memory
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>>>> 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



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