tree 013eb3685dd5341a5518c56b0aea5aa909a656b4
parent 34211ee3f2c60dc784f4d211b8c301860f5bd9a1
author snyh <snyh@snyh.org> 1544276655 +0100
committer Michael Kerrisk <mtk.manpages@gmail.com> 1544276655 +0100

syscall.2: Fix wrong retval register number in alpha architecture

alpha use v0 e.g. $0 as the return value register both in
syscall ABI and C ABI.

see also
https://github.com/torvalds/linux/blob/master/arch/alpha/kernel/entry.S#L479

The normal Alpha C ABI use a0~a5 to pass arguments and use v0 as
the return value register. See here
https://www2.cs.arizona.edu/projects/alto/Doc/local/alpha.register.html

The syscall ABI use v0 as the trap number, a0~a5 to pass arguments
and use a3 as a indicator (bool type) whether has a error occurred.

We can also see the libc's syscall wrapper implements at
https://code.woboq.org/userspace/glibc/sysdeps/unix/sysv/linux/alpha/syscall.S.html
The v0 is the normal used as return register, and we can see the
return processing doesn't do anything about a0 which is the wrong
register of currently syscall(2) description.

p.s. I found this wrong description because I'm porting Go gc to
a new CPU architecture which is similar to Alpha, And I use the
wrong register at first, then I have inspect the kernel code and
objdump to ensure the right syscall ABI.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>