Re: Emulating missing linux syscalls

[Joerg Sonnenberger <joerg%bec.de@localhost>]

Am Wed, Apr 13, 2022 at 09:51:31PM -0000 schrieb 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);

There are essentially two use cases here:
(1) I want a simple interface to transfer data from one fd to another
without extra copies.

(2) I wanto avoid copies AND I want to avoid system calls.

For the former:
    int splice(int dstfd, int srcfd, off_t *len);

is more than good enough. "Transfer up to [*len] octets from srcfd to
dstfd, updating [len] with the actually transferred amount and returning
the first error if any.

For the second category, an interface more like the posix_spawn
interface (but without all the extra allocations) would be useful.

> >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 :-)

You still provide the system call as front end, but internally implement
it on top of regular read/write to a temporary buffer.

Joerg