Re: [PATCH v6 12/20] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

Dr. Philipp Tomsich
2015-12-21T18:39:53Z

> On 18 Dec 2015, at 13:47, Arnd Bergmann <arnd@arndb.de> wrote:
>
>> 3. Follow the PCS up to glibc but always pass syscall arguments in W
>> registers, like AArch32 compat support (the least preferred option,
>> the only advantage is a single wrapper for all syscalls but it would
>> be doing unnecessary zeroing even for syscalls where it isn't needed)
>
> This would mean we cannot pass 64-bit arguments in registers, right?

Note that there’s no 32bit registers (the ‘w’-form always refers to the lower
32bits of a 64bit register, with implicit zero-extension)… and load/store
instructions always use the full base-register (‘x’-form) for address calculation.
I.e. a load/store would inadvertently pickup “random garbage” in the upper
32bits, if no explicit zero-extension is applied.

In other words: all zero-extensions for 32bit arguments should be explicit
on the kernel side.

Regards,
Philipp.