From: Stephen Rothwell <[email protected]>
Date: Tue, 7 Feb 2006 11:27:13 +1100
> *If* we do get is_compat_task(), what would be you reaction to something
> like this:
...
> + if (is_compat_task())
> + dfd = compat_sign_extend(dfd);
A load and a test is more expensive than the assembler
stubs where we _know_ we are compat.
The alternative suggestions get less and less efficient :-) My whole
desire is to optimize this as much as possible, without the overhead
of an extra stack frame or "is_compat_task()" kinds of runtime tests.
Really, the way to do this is to have a description header file, that
does stuff like:
SIGN1(STUB_NAME, REAL_SYSCALL, ARG1_TO_EXTEND)
SIGN2(STUB_NAME, REAL_SYSCALL, ARG1_TO_EXTEND, ARG2_TO_EXTEND)
etc. etc. for each syscall that only needs sign extension compat
handling, and the platform provides definitions of these macros which
expand the above as appropriate. Use arch/sparc64/kernel/sys32.S as a
guide.
So for sys_exit_group you'd list something like:
SIGN1(sys32_exit_group, sys_exit_group, ARG0)
"ARG0" would get defined to whatever register mnemonic the
first argument to a function gets passed into, ARG1 to the
second, etc. So on Sparc64 that would be:
#define ARG0 %o0
#define ARG1 %o1
#define ARG2 %o2
#define ARG3 %o3
#define ARG4 %o4
#define ARG5 %o5
and you'd use "sys32_exit_group" in the compat syscall table. The
SIGN1 definition on sparc64 would be exactly what it is right now
in that sys32.S file.
Such a header can be use to generate stubs on every platform.
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