[PATCH v1 3/3] arm64/sve: Skip flushing Z registers with 128 bit vectors

[Dave Martin]

On Mon, May 10, 2021 at 05:16:58PM +0100, Mark Brown wrote:
> On Mon, May 10, 2021 at 04:08:09PM +0100, Dave P Martin wrote:
> > On Mon, May 10, 2021 at 01:23:48PM +0100, Mark Brown wrote:
> 
> > >  SYM_FUNC_START(sve_flush_live)
> > > +	cbz		x0, 1f	// A VQ-1 of 0 is 128 bits so no extra Z state
> 
> > Should we worry about branch mispredicts here?  It may be in the noise,
> > but I wonder whether it's worth considering use of alternatives here
> > instead.
> 
> If people are happy adding an alternative we can definitely do that,
> people seemed to want to avoid them in the past and at this point I
> don't have concrete data to support how much of a win is but it seems
> very likely that it'll have the best overall performance - systems that
> only have 128 bit vectors will never have to worry about the non-shared
> bits and...
> 
> > I have a suspicion that VL = 128 bits won't be common at runtime, except
> > in the case of systems where the physical (or max usable) vector length
> > (i.e., sve_max_vl) is 128 bits.  
> 
> ...like you I expect that systems with more than 128 bits won't tend to
> configure down to 128 bits.  At the minute it's kind of finger in the
> air what the practical impact actually is though, quite a lot of
> unresolved variables.
> 
> Given the recently announced requirement for SVE in v9 I'd expect that
> we'll actually see quite a lot of 128 bit systems in the wild for at
> least some period, like with our own Neoverse N2 cores.

Agreed.  Perhaps for the longer term too, in hardware aimed at embedded
systems.

Either way, this change makes a clear place to slot an alternative into
if we later decide we want to go down that path.  So I guess I'm happy.

> > > +		unsigned long vq_minus_one =
> > > +			sve_vq_from_vl(current->thread.sve_vl) - 1;
> > > +		sve_set_vq(vq_minus_one);
> > > +		sve_flush_live(vq_minus_one);
> 
> > Seems reasonable.  sve_flush_live() could alternatively be made a C
> > function, with asm wrappers for sve_flush_{z,p,ffr} so that the
> > conditional logic can be inlined -- but I can't see that it would
> > improve the generated code much.  So I'd be happy with it to stay in
> > this form.
> 
> Yeah, I faffed a bit with options but it seemed like the effort wasn't
> going to be worth it, mainly inflating the size of the code change.

Fair enough.

Cheers
---Dave