Re: [PATCH 01/24] iov: add some functions to manage iovec

[Laurent Vivier <lvivier@redhat.com>]

On 2/6/24 17:10, Stefano Brivio wrote:
> On Fri,  2 Feb 2024 15:11:28 +0100
> Laurent Vivier <lvivier@redhat.com> wrote:
> ...
> diff --git a/iov.h b/iov.h
> new file mode 100644
> index 000000000000..31fbf6d0e1cf
> --- /dev/null
> +++ b/iov.h
> @@ -0,0 +1,46 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +
> +/* some parts copied from QEMU include/qemu/iov.h */
> +
> +#ifndef IOVEC_H
> +#define IOVEC_H
> +
> +#include <unistd.h>
> +#include <string.h>
> +
> +size_t iov_from_buf_full(const struct iovec *iov, unsigned int iov_cnt,
> +			 size_t offset, const void *buf, size_t bytes);
> +size_t iov_to_buf_full(const struct iovec *iov, const unsigned int iov_cnt,
> +		       size_t offset, void *buf, size_t bytes);
> +
> +static inline size_t iov_from_buf(const struct iovec *iov,
> +				  unsigned int iov_cnt, size_t offset,
> +				  const void *buf, size_t bytes)
> +{
> Is there a particular reason to include these two in a header? The
> compiler will inline as needed if they are in a source file.
>
This code has been introduced in QEMU by:

commit ad523bca56a7202d2498c550a41be5c986c4d33c
Author: Paolo Bonzini <pbonzini@redhat.com>
Date:   Tue Dec 22 12:03:33 2015 +0100

     iov: avoid memcpy for "simple" iov_from_buf/iov_to_buf

     memcpy can take a large amount of time for small reads and writes.
     For virtio it is a common case that the first iovec can satisfy the
     whole read or write.  In that case, and if bytes is a constant to
     avoid excessive growth of code, inline the first iteration
     into the caller.

     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
     Message-id: 1450782213-14227-1-git-send-email-pbonzini@redhat.com
     Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Is the compiler able to check "bytes" is a constant and inline the function if the 
definition is in a .c file and not in a .h ?

Thanks,
Laurent