Re: [PATCH] man2 : syscall.2 : add notes

[Mike Frysinger <vapier-aBrp7R+bbdUdnm+yROfE0A@public.gmane.org>]

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On Monday 01 April 2013 05:30:06 Michael Kerrisk (man-pages) wrote:
> On Mon, Apr 1, 2013 at 10:37 AM, Mike Frysinger <vapier-aBrp7R+bbdUdnm+yROfE0A@public.gmane.org> wrote:
> > On Monday 01 April 2013 03:19:43 Mike Frysinger wrote:
> >> for ARM OABI, there is no such padding, and the proposed example is
> >> wrong and will not work.
> >> 
> >> for ARM EABI, the ABI requires that 64bit values be passed in register
> >> pairs. since the kernel people wanted to avoid an assembly trampoline to
> >> 
> >> unpack the 64bit value with EABI, you have to call it as proposed:
> >>       syscall(readahead, fd, _pad, high32, low32)
> >> 
> >> for MIPS, only the O32 ABI has this behavior.
> >> 
> >> for PPC, only the 32bit ABI has this behavior.
> >> 
> >> otherwise, i don't believe anyone else does this -- they just pass
> >> things along in registers w/out padding.
> > 
> > in random grepping of code bases (uClibc), i believe the xtensa arch also
> > does 64bit register pair aligning.  a cursory scan of the kernel seems
> > to back this up.
> 
> Also SuperH?

i don't think so ... the pread/pwrite is indeed funky for SuperH, but i'm 
pretty sure that's a wart they accidentally copied from another arch (ppc 
maybe?) when they implemented the syscall rather than needing to do 64bit 
register alignment.  i say that because qemu/strace/glibc don't do the 
register realigning for any other function.

> For my own education: which part of the kernel sources backed this up?

for xtensa, this part:
	arch/xtensa/include/uapi/asm/unistd.h:
	__SYSCALL(260, sys_readahead, 5)
that says readahead takes 5 args, but that's only true for 32bit arches if 
you're re-aligning the value.  the other 64bit syscalls have the same property 
(+1 to the normal # of args).

additionally, the arch/xtensa/kernel/syscall.c file has a custom fadvise64_64 
syscall with re-order arguments (with "advice" moved from last to 2nd) so that 
the shifting of args doesn't end up requiring 7 slots (ala mips/o32).
-mike

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