On Fri, Jan 27, 2023, at 20:46, Johannes Weiner wrote:

On Fri, Jan 27, 2023 at 04:46:38PM +0100, Arnd Bergmann wrote:

quoted

- if you make it a 32-bit type, this breaks calling it from
  normal userspace that defines off_t as a 64-bit type

- if you change it to a 64-bit loff_t, there are three
  separate calling conventions for 64-bit, 32-bit with
  aligned register pairs and other 32-bit, plus you
  exceed the usual limit of six system call arguments

That's a good point, thanks for raising it, Arnd.

quoted

A separate problem may be the cstat_version argument, usually
we don't use interface versions but instead use a new
system call number if something changes in an incompatible
way.

I suppose from a userspace POV, a version argument vs calling a
separate syscall doesn't make much of a difference. So going with
loff_t and dropping cstat_version seems like a sensible way forward.

As an added bonus, versioning the syscall itself means the signature
can change in v2. This allows dropping the unused flags arg for now.

That would leave it at:

  int cachestat(unsigned int, loff_t, size_t len, struct cachestat *cstat);

There is still a problem of incompatible calling conventions:
on architectures that require even/odd register pairs, this would
end up like

int cachestat(unsigned int, long unused, u32 off_low, u32 off_high,
              size_t len, struct cachestat *cstat);

A more portable way to do this would be to pass the offset by
reference, but that makes it a bit awkward in userspace.

Or the arguments could be rearranged to put the low/high argument
pair first/second, third/fourth or fifth/sixth argument, at least
on the kernel ABI to avoid having another situation like
sys_arm_fadvise64_64.

and should we ever require extensions - new fields, flags - they would
come through a new cachestat2().

Would anybody have objections to that?

If there is room for another argument, I would keep the 'flags'
as a way for extending in a compatible way, that is pretty standard
now, just not flags plus version.

      Arnd