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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 22 Mar 2018 17:44:05 +0000
Message-ID: <CALCETrX6H4wO-sFhpVCjw-x2gZUagohO+_dNkcf0X_zy41qUCg@mail.gmail.com>
Subject: Re: [RFC PATCH 0/3] kernel: add support for 256-bit IO access
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Laight <David.Laight@aculab.com>,
        Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>,
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On Thu, Mar 22, 2018 at 5:40 PM, Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
> On Thu, Mar 22, 2018 at 10:33:43AM +0100, Ingo Molnar wrote:
>>
>>  - I think the BPF JIT, whose byte code machine languge is used by an
>>    increasing number of kernel subsystems, could benefit from having vector ops.
>>    It would possibly allow the handling of floating point types.
>
> this is on our todo list already.
> To process certain traffic inside BPF in XDP we'd like to have access to
> floating point. The current workaround is to precompute the math and do
> bpf map lookup instead.
> Since XDP processing of packets is batched (typically up to napi budget
> of 64 packets at a time), we can, in theory, wrap the loop with
> kernel_fpu_begin/end and it will be cleaner and faster,
> but the work hasn't started yet.
> The microbenchmark numbers you quoted for xsave/xrestore look promising,
> so we probably will focus on it soon.
>
> Another use case for vector insns is to accelerate fib/lpm lookups
> which is likely beneficial for kernel overall regardless of bpf usage.
>

This is a further argument for the deferred restore approach IMO.
With deferred restore, kernel_fpu_begin() + kernel_fpu_end() has very
low amortized cost as long as we do lots of work in the kernel before
re-entering userspace.  For XDP workloads, this could be a pretty big
win, I imagine.

Someone just needs to do the nasty x86 work.