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SubjectRe: [PATCH] compatibility syscall layer (lets try again)
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   From: "Martin Schwidefsky" <schwidefsky@de.ibm.com>
Date: Mon, 9 Dec 2002 18:16:43 +0100

>Architecture maintainers, can you comment on how easy/hard it is to do the
>same thing on your architectures? I _assume_ it's trivial (akin to the
>three-liner register state change in i386/kernel/signal.c).

For s390/s390x this is actually quite tricky. The system call number is
coded in the instruction, e.g. 0x0aa2 is svc 162 or sys_nanosleep. There
is no register involved that contains the system call number I could
simply change. I either have to change the instruction (no way) or I
have to avoid going back to userspace in this case. This would require
assembler magic in entry.S. Not nice.

Put the magic restart_block syscall at some fixed place in every
user process, change the PC to that. Or, alternatively, put the
restart_block syscall insn on the stack and point the PC at that.

This isn't rocket science :-)

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