Re: [PATCH 2/4] aarch64: Re-implement setcontext without sigreturn syscall

[Will Newton <will dot newton at linaro dot org>]

On 17 March 2014 17:48, Marcus Shawcroft <marcus.shawcroft@gmail.com> wrote:
> On 13 March 2014 10:45, Will Newton <will.newton@linaro.org> wrote:
>
>
>> +       /* Restore the general purpose registers.  */
>> +       mov     x0, x9
>
> Looks like this code is treating x9 as callee saved over the kernel
> call. While this is true with the current implementation of the
> kernel, the glibc port for AArch64 currently treats only the argument
> registers as preserved.  This is a hang over from the early days of
> the AArch64 port when the kernel guys wanted the glibc port to be
> conservative in this respect in order that they have the opportunity
> to be selective in what was restored on exit from the kernel.

Really, is that true? It seems to me you would have to save/restore in
userland anyway so I am not sure what would actually be saved by
making syscalls behave so differently from function calls.

> Catalin, Can you comment on the above paragraph?  Is it still
> important that glibc / user space be conservative in its assumptions
> about which registers are preserved over system call or can we now
> relax this position and exploit the fact that the kernel preserves
> x9-x30 over kernel calls?
>
>
>> +       cfi_def_cfa(x0, 0)
>
> Space before (....

Thanks, I fixed that.

-- 
Will Newton
Toolchain Working Group, Linaro