Re: [PATCH 5/9] migration/qemu-file: Add qemu_file_get_to_fd()

[Juan Quintela <quintela@redhat.com>]

Avihai Horon <avihaih@nvidia.com> wrote:
> Add new function qemu_file_get_to_fd() that allows reading data from
> QEMUFile and writing it straight into a given fd.
>
> This will be used later in VFIO migration code.
>
> Signed-off-by: Avihai Horon <avihaih@nvidia.com>
> ---
>  migration/qemu-file.c | 34 ++++++++++++++++++++++++++++++++++
>  migration/qemu-file.h |  1 +
>  2 files changed, 35 insertions(+)
>
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index 1479cddad9..cad3d32eb3 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -867,3 +867,37 @@ QIOChannel *qemu_file_get_ioc(QEMUFile *file)
>  {
>      return file->has_ioc ? QIO_CHANNEL(file->opaque) : NULL;
>  }
> +
> +/*
> + * Read size bytes from QEMUFile f and write them to fd.
> + */
> +int qemu_file_get_to_fd(QEMUFile *f, int fd, size_t size)
> +{
> +    while (size) {
> +        size_t pending = f->buf_size - f->buf_index;
> +        ssize_t rc;
> +
> +        if (!pending) {
> +            rc = qemu_fill_buffer(f);
> +            if (rc < 0) {
> +                return rc;
> +            }
> +            if (rc == 0) {
> +                return -1;
> +            }
> +            continue;
> +        }
> +
> +        rc = write(fd, f->buf + f->buf_index, MIN(pending, size));
> +        if (rc < 0) {
> +            return rc;
> +        }
> +        if (rc == 0) {
> +            return -1;
> +        }
> +        f->buf_index += rc;
> +        size -= rc;
> +    }
> +
> +    return 0;
> +}

Is there a really performance difference to just use:

uint8_t buffer[size];

qemu_get_buffer(f, buffer, size);
write(fd, buffer, size);

Or telling it otherwise, what sizes are we talking here?

Thanks, Juan.


> diff --git a/migration/qemu-file.h b/migration/qemu-file.h
> index 3f36d4dc8c..dd26037450 100644
> --- a/migration/qemu-file.h
> +++ b/migration/qemu-file.h
> @@ -162,6 +162,7 @@ int qemu_file_shutdown(QEMUFile *f);
>  QEMUFile *qemu_file_get_return_path(QEMUFile *f);
>  void qemu_fflush(QEMUFile *f);
>  void qemu_file_set_blocking(QEMUFile *f, bool block);
> +int qemu_file_get_to_fd(QEMUFile *f, int fd, size_t size);
>  
>  void ram_control_before_iterate(QEMUFile *f, uint64_t flags);
>  void ram_control_after_iterate(QEMUFile *f, uint64_t flags);