On Monday 21 December 2015, Dr. Philipp Tomsich wrote:

quoted

On 18 Dec 2015, at 13:47, Arnd Bergmann [off-list ref] wrote:

quoted

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.

I think that is what Catalin meant with the single wrapper in the description:
the kernel always zeroes out the upper 32 bits in the initial trap, and then
we only need to do sign-extensions for the few cases that need it, and pass 64-bit
arguments in two lower halves. In contrast, approach 1 adds a separate wrapper
for each syscall that takes care of both sign-extending where necessary and
zero-extending all othe 32-bit arguments but not the 64-bit arguments.

	Arnd