> 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
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