Re: [PATCH v1 2/3] kselftest/arm64: Validate vector lengths are set in sve-probe-vls

[Dave Martin <Dave.Martin@arm.com>]

On Wed, Jul 28, 2021 at 12:07:01PM +0100, Mark Brown wrote:
> On Wed, Jul 28, 2021 at 10:41:51AM +0100, Dave Martin wrote:
> 
> > This test was originally a diagnostic tool, so the way VLs are probed
> > aims to be efficient, rather than being defensive against the kernel
> > doing weird stuff.
> 
> Yeah, the whole probing thing doesn't really fit into kselftest's idea
> of what a test is - I just put this in here since it seemed like a cheap
> extra validation that we could add in with little bother rather than
> because it's a complete and thorough validation of every possible thing
> that could go wrong.  I'd be just as happy to not modify this at all but
> since it does try the intermediate VLs it didn't seem like a terrible
> idea to throw in some validation while we're at it.
> 
> > If the kernel returns a vl > than the one we tried to set, we'll end
> > up in an infinite loop because of the way the loop deliberately uses the
> > kernel's return value to skip unsupported VLs.  So, it might be better
> > to probe every single architecturally possible VL and sanity check
> > everything instead.
> 
> Yup, that'd obviously be a complete rewrite though.  We'd not only need
> to probe every possible vector length, but also validate that any
> unsupported vector lengths report the expected vector length instead
> when we try them.

Fair enough, but is it worth dropping a comment in to clarify what this
does and doesn't test?  This could be an area for future improvement.

> > > +		if (rdvl_sve() != vl)
> > > +			ksft_exit_fail_msg("Set VL %d, RDVL %d\n",
> > > +					   vl, rdvl_sve());
> 
> > If this fails, the VL vl wasn't "Set" at all.  I found this a bit
> > confusing from just looking at this hunk.
> 
> > Can we write something like "PR_SVE_SET_VL reports VL %d, RDVL %d"?
> 
> Sure.
> 
> > I'm not sure of the correct option for forcing SVE off against the
> > compiler default -- check with the tools folks.  If there isn't a
> > proper way to do this, it's a toolchain defect so we should flag it up,
> > but -mgeneral-regs-only might work for us even though it's a bit of a
> > sledgehammer.
> 
> -march should do the trick (if it doesn't the base kernel already has
> issues).  This is a separate issue that affects all the arm64 selftests
> I think.

OK, if there's already precedent then I guess we can go with that.

Cheers
---Dave