Re: [glibc] preadv/pwritev question

[Chris Metcalf <cmetcalf@mellanox.com>]

On 5/31/2016 4:04 PM, Yury Norov wrote:
> Hi Chris,
>
> In path a63c7fa18a (Add sysdeps/unix/sysv/linux/generic/.) you add
> this:
> +++ b/sysdeps/unix/sysv/linux/generic/wordsize-32/preadv.c
>
> [...]
>
> +static ssize_t
> +do_preadv (int fd, const struct iovec *vector, int count, off_t
> offset)
> +{
> +  assert (sizeof (offset) == 4);
> +  return INLINE_SYSCALL (preadv, __ALIGNMENT_COUNT (5, 6), fd,
> +                         vector, count, __ALIGNMENT_ARG
> +                         __LONG_LONG_PAIR (offset >> 31, offset));
> +}
> +
>
> And this is the code that is picked up if I choose wordsize-32 for my
> AARCH64/ILP32. So I have questions.
>
> 1. What is the assert for? We agreed that all new ABIs will be 64-bit
> off_t only.
>
> I fixed it internally like this:
> +#ifndef __OFF_T_MATCHES_OFF64_T
>          assert (sizeof (offset) == 4);
> +#endif
>
> There is a bunch of similar assertions in glibc.
>
> 2. This one looks weird:
> __LONG_LONG_PAIR (offset >> 31, offset))
> Why 31-bit offset? And why you don't mask 2nd argument?
> Later in your patch I see this:
> +static ssize_t
> +do_preadv64 (int fd, const struct iovec *vector, int count, off64_t
> offset)
>
> +{
> +  return INLINE_SYSCALL (preadv, __ALIGNMENT_COUNT (5, 6), fd,
> +                         vector, count, __ALIGNMENT_ARG
> +                         __LONG_LONG_PAIR ((off_t) (offset >> 32),
> +                                           (off_t) (offset & 0xffffffff)));
> +}
>
> And it looks correct to me. If 1st version is correct as well, I think
> it should be commented.

I did this work before x32 came out, so I tried to model it more closely on
the existing x86 compat API.  I agree that a 64-bit off_t model seems reasonable;
however, the code does exactly what it does to match x86, namely preadv() takes
a 32-bit offset, and preadv64() take a 64-bit offset.  The assert() in preadv to force
sizeof to be 4 is exactly why in that routine we use (offset >> 31, offset).  Since
we know offset fits in 32 bits, all we need to do is properly sign-extend it into
64 bits in the high register of the pair, which is what (offset >> 31) does - you end
up with only 0 or -1, thus sign-extending the 32-bit signed off_t. Then in
preadv64() we actually need to break apart the 64-bit offset into a high 32 bits
and a low 32 bits, which is what (offset >> 32, offset & 0xffffffff) does.

For a 64-bit off_t you will want to not compile preadv.c at all, and instead make
__libc_preadv() and friends be aliases of __libc_preadv64().

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com