Re: [Qemu-devel] [Qemu-block] [RFC PATCH] block: local qiov helper

[Stefan Hajnoczi]

On Fri, Jan 25, 2019 at 07:46:01PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> Hi all.
> 
> What about such a simple helper for a very often patter around
> qemu_iovec_init_external ?

Sounds good, qemu_iovec_init() has 55 references vs
qemu_iovec_init_external() with 51.  It's worth making
qemu_iovec_init_external() nicer to use.

> If we like it, I'll update other callers of qemu_iovec_init_external.
> 
> Possible interface change would be
> LOCAL_QIOV(lc, buf, len);
> instead of 
> LocalQiov lc = LOCAL_QIOV(lc, buf, len);
> 
> or, may be, someone has a better idea?

Bike-shedding territory, but I prefer LocalQiov lc = LOCAL_QIOV(lc, buf,
len) because it reveals the type.  This makes the code easier to read
than just LOCAL_QIOV(lc, buf, len) by itself - the reader is forced to
look up the macro definition to figure out what magic happens.

> diff --git a/block/io.c b/block/io.c
> index bd9d688f8b..c7d7b199c1 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -949,18 +949,13 @@ int bdrv_preadv(BdrvChild *child, int64_t offset, 
> QEMUIOVector *qiov)
>  
>  int bdrv_pread(BdrvChild *child, int64_t offset, void *buf, int bytes)
>  {
> -    QEMUIOVector qiov;
> -    struct iovec iov = {
> -        .iov_base = (void *)buf,
> -        .iov_len = bytes,
> -    };
> +    LocalQiov lq = LOCAL_QIOV(lq, buf, bytes);
>  
>      if (bytes < 0) {
>          return -EINVAL;
>      }
>  
> -    qemu_iovec_init_external(&qiov, &iov, 1);
> -    return bdrv_preadv(child, offset, &qiov);
> +    return bdrv_preadv(child, offset, &lq.qiov);

I think it's unfortunate that LocalQiov is necessary since the caller
only needs the qiov.  Can we afford to embed the struct iovec into
QEMUIOVector?

That way callers don't need a separate LocalQiov type:

  QEMUIOVector qiov = QEMU_IOVEC_INIT_BUF(qiov, buf, bytes);
  ...
  return bdrv_preadv(child, offset, &qiov);

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