Re: [Qemu-devel] [PATCH 2/3] linux-user: added support for preadv() system call.

[Peter Maydell]

On 6 October 2016 at 14:59, Dejan Jovicevic <address@hidden> wrote:
> This system call performs the same task as the readv system call,
> with the exception of having the fourth argument, offset, which
> specifes the file offset at which the input operation is to be performed.
>
> This implementation is based on the existing readv implementation.
>
> Signed-off-by: Dejan Jovicevic <address@hidden>
> ---
>  linux-user/syscall.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 0815f30..10b2552 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -9894,6 +9894,19 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
> arg1,
>              }
>          }
>          break;
> +#if defined(TARGET_NR_preadv) && defined(CONFIG_PREADV)
> +    case TARGET_NR_preadv:
> +        {
> +            struct iovec *vec = lock_iovec(VERIFY_WRITE, arg2, arg3, 0);
> +            if (vec != NULL) {
> +                ret = get_errno(preadv(arg1, vec, arg3, arg4));
> +                unlock_iovec(vec, arg2, arg3, 1);
> +            } else {
> +                ret = -host_to_target_errno(errno);
> +           }
> +        }
> +        break;
> +#endif

preadv is an "interruptible syscall", meaning that it can
return EINTR if a signal occurs while it is blocking. This
means that in QEMU we need to implement it via the
safe_syscall() wrapper (see the long explanatory comment
in linux-user/qemu.h). The same applies to pwritev. See
the implementations of TARGET_NR_readv and TARGET_NR_writev
for an example of how to do this; basically you just need
a safe_syscall4() line to define safe_preadv(), and then
use it instead of direct preadv(). As a side-benefit you'll
then be able to drop the configure patch since we don't
go via libc.

thanks
-- PMM