Re: [RFC PATCH 0/3] kernel: add support for 256-bit IO access

On Thu, Mar 22, 2018 at 5:40 PM, Alexei Starovoitov
[off-list ref] wrote:

On Thu, Mar 22, 2018 at 10:33:43AM +0100, Ingo Molnar wrote:

quoted

 - 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.