Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()

[Arjun Roy]

On Tue, Apr 13, 2021 at 2:19 PM David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> > If we're special-casing 64-bit architectures anyways - unrolling the
> > 32B copy_from_user() for struct rseq_cs appears to be roughly 5-10%
> > savings on x86-64 when I measured it (well, in a microbenchmark, not
> > in rseq_get_rseq_cs() directly). Perhaps that could be an additional
> > avenue for improvement here.
>
> The killer is usually 'user copy hardening'.
> It significantly slows down sendmsg() and recvmsg().
> I've got measurable performance improvements by
> using __copy_from_user() when the buffer since has
> already been checked - but isn't a compile-time constant.
>
> There is also scope for using _get_user() when reading
> iovec[] (instead of copy_from_user()) and doing all the
> bound checks (etc) in the loop.
> That gives a measurable improvement for writev("/dev/null").
> I must sort those patches out again.
>
>         David
>

In this case I mean replacing copy_from_user(rseq_cs, urseq_cs,
sizeof(*rseq_cs)) with  4 (x8B=32 total) unsafe_get_user() (wrapped in
user_read_access_begin/end()) which I think would just bypass user
copy hardening (as far as I can tell).

-Arjun

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