Re: [PATCH v12 06/40] arm64/sme: Provide ABI documentation for SME

[Szabolcs Nagy <Szabolcs.Nagy@arm.com>]

The 03/11/2022 18:42, Mark Brown wrote:
> On Fri, Mar 11, 2022 at 05:21:21PM +0000, Szabolcs Nagy wrote:
> > The 02/25/2022 16:58, Mark Brown wrote:
> > > +* On creation fork() or clone() the newly created process will have PSTATE.SM
> > > +  and PSTATE.ZA cleared.
> 
> > is there a reason why fork() clears ZA?
> 
> > i think this is a minor issue, but the usual expectation is that
> > on thread creation thread local state is reset in the child, but
> > in a forked child the state is the same as in the parent (where
> > ZA is preserved according to the first rule).
> 
> It was partly consistency with SM and the SVE state (though that is also
> covered by just being in a system call unlike ZA) and partly concerns
> about what happens if the fork() happens in library code which isn't SME
> aware - it would end up carrying around a copy of ZA with associated
> power and performance impacts if it doesn't exec().  Overall it seemed
> like there would to be less potential for unpleasant surprises if we
> consistently discard the data.
> 
> That's not a *super* strongly held opinion though, we could switch to
> preserving whenever we preserve TPIDR2.

i think it's slightly better to treat ZA like TPIDR2,
so only clear if CLONE_SETTLS is set.

otherwise in principle the child can return to the frame
where ZA was used and expect it to work (it's hard to
come up with a reason why would some code do that, but
this is valid in a single-threaded fork child).

sorry for not deciding this earlier.
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