[Qemu-devel] alpha - syscalls can return two values

[Qemu-devel] alpha - syscalls can return two values

[Vince Weaver]

Hello

On the Alpha architecture, system calls can return two values, one in 
register $0 and one in register $1.  This is unusual, so there is no 
mechanism I can see in linux-user/syscall.c for handling this properly.

What would be the best way to handle this?

This is needed to implement the getxuid and getxgid syscalls.  Without 
these, the spec2k perlbmk fails to run.

Vince

Re: [Qemu-devel] alpha - syscalls can return two values

[Vince Weaver]

and as soon as I sent this, I realized I was reading the code wrong.

It doesn't return the second result in $1, but rather it returns the 
second result on the stack at offset 80.

This remains a problem though, as I don't think there's an easy way to get 
the stack pointer from the syscall code.

The relevant linux-kernel syscall code can be found in
   ./arch/alpha/kernel/entry.S

specifically the getxuid, getxgid, getxpid, syscalls.

Vince

Re: [Qemu-devel] alpha - syscalls can return two values

[Vince Weaver]

and to keep correcting myself, it seems like the Alpha multi-value
return syscalls do not in fact use the stack at all.

I'm just posting this to clear up what I posted earlier.

What happens is:
   normal return value goes into $v0 (as always)
   errno is put into $a3 (as always)
   secondary return value is placed into $a4.

I have a patch that implements this that I'll send in a separate e-mail.

Vince