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fork: Allow ABI to specify fork return values for child.

Description

fork: Allow ABI to specify fork return values for child.

At least Linux x86 ABI's does not use carry bit and expects that the dx register
is preserved. For this add a new sv_set_fork_retval hook and call it from cpu_fork().

Add a short comment about touching dx in x86_set_fork_retval(), for more details
see phab comments from kib@ and imp@.

Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D31472
MFC after: 2 weeks

Details

Provenance
dchaginAuthored on Aug 12 2021, 8:45 AM
Reviewer
kib
Differential Revision
D31472: fork: Touch the registers specific to FreeBSD under the condition.
Parents
R10:fc37be246021: linux(4): Plug in aarch64 fcntl flags.
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