The 03/11/2022 18:42, Mark Brown wrote:

On Fri, Mar 11, 2022 at 05:21:21PM +0000, Szabolcs Nagy wrote:

quoted

The 02/25/2022 16:58, Mark Brown wrote:

quoted

+* On creation fork() or clone() the newly created process will have PSTATE.SM
+  and PSTATE.ZA cleared.

quoted

is there a reason why fork() clears ZA?

quoted

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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