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

[Eric Dumazet]

On Wed, Apr 14, 2021 at 9:55 AM David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Arjun Roy
> > Sent: 13 April 2021 23:04
> >
> > 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).
>
> 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.


On which arch are you looking at ?

On x86_64 at least, code generation is just perfect.
Not even a conditional jmp, it is all handled by exceptions (if any)

stac
copy();
copy();
clac


<out_of_line>
efault_end: do error recovery.



>
> 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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