On Monday 21 December 2015, Catalin Marinas wrote:

On Fri, Dec 18, 2015 at 01:47:55PM +0100, Arnd Bergmann wrote:

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On Friday 18 December 2015 11:42:19 Catalin Marinas wrote:

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2. Follow the PCS up to glibc and get glibc to zero the top part (not
   always safe with hand-written assembly, though we already do this for
   AArch32 where the PCS only specifies 4 arguments in registers, the
   rest go on the stack)

I assume this needs special handling for syscalls with 64-bit arguments
in both glibc and kernel.

I think glibc only should suffice, if it is its responsibility to zero
the top 32-bit part.

The kernel still needs to know about whether to call e.g. sys_llseek or
sys_lseek. The default syscall table contains llseek for 32-bit architectures,
but the current patch set uses lseek because that makes more sense when
you have 64-bit registers.

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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?

Not in a single register but two (like we do on AArch32).

Yes, that's what I mean. Essentially we'd use the unmodified 32-bit API
here.

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My preference, as stated above, is (1). You can write the wrappers in C
directly and let the compiler upgrade the types when calling the native
syscall. But any other option would be fine (take some inspiration from
other architectures). Unfortunately we don't have COMPAT_SYSCALL_DEFINE
for all functions that we need to wrap, it would have been easier (so we
need to add them but probably in the arch/arm64 code).

It would be nice to have that code architecture-independent, so we can
share it with s390 and only need to update one place when new syscalls
get added.

We could indeed move things like:

COMPAT_SYSCALL_DEFINE3(s390_read, unsigned int, fd, char __user *, buf, compat_size_t, count)

to the core code and share them between s390 and arm64/ILP32. So let's
stick to option 1.

Ok.

	Arnd