Re: [PATCH v2 04/11] io: add pwritev support to QIOChannelFile

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Oct 10, 2022 at 04:34:01PM +0300, Nikolay Borisov wrote:
> The upcoming 'fixed-ram' feature would require qemu to write data at
> specific offsets of the file. This is currently missing so this patch
> adds it. I've chosen to implement it as a type-specific function rather
> than plumb it through the generic channel interface as it relies on
> being able to do seeks.

Well the base QIOChannel does have  an 'io_seek' callback, so it
already has some support for seeking.

In addition to this QIOChannelFile class, the QIOChannelBuffer
and QIOChannelNull classes could also allow seekable IO ops.

So I think it is OK to add this to the QIOChannel base class,
especially since your next patch adds a "SEEKABLE" feature
flag.

We have  io_writev/io_readv callbacks. If we add
io_pwritev/io_preadv callbacks too, we should validate that
they exist when the SEEKABLE feature is marked present.

> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  include/io/channel-file.h |  5 +++++
>  io/channel-file.c         | 24 ++++++++++++++++++++++++
>  2 files changed, 29 insertions(+)
> 
> diff --git a/include/io/channel-file.h b/include/io/channel-file.h
> index 50e8eb113868..0a5d54f5e58e 100644
> --- a/include/io/channel-file.h
> +++ b/include/io/channel-file.h
> @@ -89,4 +89,9 @@ qio_channel_file_new_path(const char *path,
>                            mode_t mode,
>                            Error **errp);
>  
> +ssize_t qio_channel_file_pwritev(QIOChannel *ioc,
> +				 const struct iovec *iov,
> +				 size_t niov,
> +				 off_t offset,
> +				 Error **errp);
>  #endif /* QIO_CHANNEL_FILE_H */
> diff --git a/io/channel-file.c b/io/channel-file.c
> index b67687c2aa64..da17d0a11ba7 100644
> --- a/io/channel-file.c
> +++ b/io/channel-file.c
> @@ -136,6 +136,30 @@ static ssize_t qio_channel_file_writev(QIOChannel *ioc,
>      return ret;
>  }
>  
> +ssize_t qio_channel_file_pwritev(QIOChannel *ioc,
> +				 const struct iovec *iov,
> +				 size_t niov,
> +				 off_t offset,
> +				 Error **errp)
> +{
> +    QIOChannelFile *fioc = QIO_CHANNEL_FILE(ioc);
> +    ssize_t ret;
> +
> + retry:
> +    ret = pwritev(fioc->fd, iov, niov, offset);
> +    if (ret <= 0) {
> +        if (errno == EAGAIN) {
> +            return QIO_CHANNEL_ERR_BLOCK;
> +        }
> +        if (errno == EINTR) {
> +            goto retry;
> +        }
> +        error_setg_errno(errp, errno, "Unable to write to file");
> +        return -1;
> +    }
> +    return ret;
> +}
> +
>  static int qio_channel_file_set_blocking(QIOChannel *ioc,
>                                           bool enabled,
>                                           Error **errp)
> -- 
> 2.34.1
> 

With regards,
Daniel
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