Re: Emulating missing linux syscalls

[christos%astron.com@localhost (Christos Zoulas)]

In article <Ylcr/ndudNE2AeHK%bec.de@localhost>, Joerg Sonnenberger  <joerg%bec.de@localhost> wrote:
>Am Tue, Apr 12, 2022 at 04:56:05PM -0000 schrieb Christos Zoulas:

>splice(2) as a concept is much older than the current Linux implementation.
>There is no reason why zero-copying for sockets should require a
>different system call for zero-copying from/to pipes. There are valid
>reasons for other combinations, too. Consider /bin/cp for example.

You don't need two system calls because the kernel knows the type of
the file descriptors and can dispatch to different implementations.
One of the questions is do you provide the means to pass an additional
header/trailer to the output data like FreeBSD does for its sendfile(2)
implementation?

int
splice(int infd, off_t *inoff, int outfd, off_t *outoff, size_t len, 
    const struct {
	    struct iov *head;
	    size_t headcnt;
	    struct iov *tail;
	    size_t tailcnt;
    } *ht, int flags);

>I was saying that the Linux system call can be implemented without a
>kernel backend, because I don't consider zero copy a necessary part of
>the interface contract. It's a perfectly valid, if a bit slower
>implementation to do allocate a kernel buffer and do IO via that.

Of course, but how do you make an existing binary use it? LD_PRELOAD
a binary to override the symbol in the linux glibc? By that logic you
don't need an in kernel linux emulation, you can do it all in userland :-)

christos