[PATCH v2] kselftest/arm64: Exit streaming mode after collecting signal context

[Mark Brown]

On Thu, Jul 20, 2023 at 11:31:13AM +0100, Will Deacon wrote:
> On Wed, Jul 12, 2023 at 12:02:30PM +0100, Mark Brown wrote:

> > +	/*
> > +	 * This is a memset() but we don't want the compiler to
> > +	 * optimise it into either instructions or a library call
> > +	 * which might be incompatible with streaming mode.
> > +	 */
> > +	for (i = 0; i < td->live_sz; i++) {
> > +		asm volatile("nop"
> > +			     : "+m" (*dest_uc)
> > +			     :
> > +			     : "memory");

> I don't think it's save to use "+m" here, since the compiler can assume
> that the address is used exactly once in the asm. If a post-indexed
> addressing mode is generated, then you can end up with register corruption.

> Stepping back, why not use either barrier() or OPTIMIZER_HIDE_VAR()
> instead?

That should work.  I was mostly just open coding OPTIMIZER_HIDE_VAR()
and noticed that memory constraints were a thing.

> The most robust fix would be to write all of the streaming mode code in
> asm, but I can appreciate that's a tonne of work for a testcase.

It's probably more proportionate to add a dependency on toolchain
support for SME, but that'd mean we hardly ever run the tests.
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