On Tue, 17 Dec 2002, Linus Torvalds wrote:
>
> Hmm.. Which system calls have all six parameters? I'll have to see if I
> can find any way to make those use the new interface too.
The only ones I found from a quick grep are
- sys_recvfrom
- sys_sendto
- sys_mmap2()
- sys_ipc()
and none of them are of a kind where the system call entry itself is the
biggest performance issue (and sys_ipc() is deprecated anyway), so it's
probably acceptable to just use the old interface for them.
One other alternative is to change the calling convention for the
new-style system call, and not have arguments in registers at all. We
could make the interface something like
- %eax contains system call number
- %edx contains pointer to argument block
- call *syscallptr // trashes all registers
and then the old "compatibility" function would be something like
movl 0(%edx),%ebx
movl 4(%edx),%ecx
movl 12(%edx),%esi
movl 16(%edx),%edi
movl 20(%edx),%ebp
movl 8(%edx),%edx
int $0x80
ret
while the "sysenter" interface would do the loads from kernel space.
That would make some things easier, but the problem with this approach is
that if you have a single-argument system call, and you just pass in the
stack pointer offset in %edx directly, then the system call stubs will
always load 6 arguments, and if we're just at the end of the stack it
won't actually _work_. So part of the calling convention would have to be
the guarantee that there is stack-space available (should always be true
in practice, of course).
Linus
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This archive was generated by hypermail 2b29 : Mon Dec 23 2002 - 22:00:17 EST