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

[David Laight <David.Laight@ACULAB.COM>]

From: Arjun Roy
> Sent: 13 April 2021 23:04
> 
> On Tue, Apr 13, 2021 at 2:19 PM David Laight <David.Laight@aculab.com> 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).

Yes that is one advantage over any of the get_user() calls.
You also lose all the 'how shall we optimise this' checks
in copy_from_user().

Repeated unsafe_get_user() calls are crying out for an optimisation.
You get something like:
	failed = 0;
	copy();
	if (failed) goto error;
	copy();
	if (failed) goto error;
Where 'failed' is set by the fault handler.

This could be optimised to:
	failed = 0;
	copy();
	copy();
	if (failed) goto error;
Even if it faults on every invalid address it probably
doesn't matter - no one cares about that path.

I've not really looked at how it could be achieved though.

It might be that the 'asm goto with outputs' variant
manages to avoid the compare and jump.

	David

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