Re: [Qemu-devel] [PATCH 3/3] dataplane: handle misaligned virtio-blk requests

[Kevin Wolf]

Am 10.01.2013 17:48, schrieb Stefan Hajnoczi:
> O_DIRECT on Linux has alignment requirements on I/O buffers and
> misaligned requests result in -EINVAL.  The Linux virtio_blk guest
> driver usually submits aligned requests so I forgot to handle misaligned
> requests.
> 
> It turns out that virtio-win guest drivers submit misaligned requests.
> Handle them using a bounce buffer that meets alignment requirements.
> 
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  hw/dataplane/virtio-blk.c | 36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/hw/dataplane/virtio-blk.c b/hw/dataplane/virtio-blk.c
> index a6696b8..991ab5f 100644
> --- a/hw/dataplane/virtio-blk.c
> +++ b/hw/dataplane/virtio-blk.c
> @@ -34,6 +34,8 @@ typedef struct {
>      struct iocb iocb;               /* Linux AIO control block */
>      QEMUIOVector *inhdr;            /* iovecs for virtio_blk_inhdr */
>      unsigned int head;              /* vring descriptor index */
> +    void *bounce_buffer;            /* used if guest buffers are unaligned */
> +    QEMUIOVector *read_qiov;        /* for read completion /w bounce buffer 
> */
>  } VirtIOBlockRequest;
>  
>  struct VirtIOBlockDataPlane {
> @@ -89,6 +91,15 @@ static void complete_request(struct iocb *iocb, ssize_t 
> ret, void *opaque)
>  
>      trace_virtio_blk_data_plane_complete_request(s, req->head, ret);
>  
> +    if (req->read_qiov) {
> +        assert(req->bounce_buffer);
> +        qemu_iovec_to_buf(req->read_qiov, 0, req->bounce_buffer, len);

Shouldn't it be qemu_iovec_from_buf()?

Makes me wonder if qtest cases might be doable...

> +        qemu_iovec_destroy(req->read_qiov);
> +        g_slice_free(QEMUIOVector, req->read_qiov);
> +    }
> +
> +    qemu_vfree(req->bounce_buffer);
> +
>      qemu_iovec_from_buf(req->inhdr, 0, &hdr, sizeof(hdr));
>      qemu_iovec_destroy(req->inhdr);
>      g_slice_free(QEMUIOVector, req->inhdr);
> @@ -136,6 +147,29 @@ static int do_rdwr_cmd(VirtIOBlockDataPlane *s, bool 
> read,
>                         QEMUIOVector *inhdr)
>  {
>      struct iocb *iocb;
> +    QEMUIOVector qiov;
> +    struct iovec bounce_iov;
> +    void *bounce_buffer = NULL;
> +    QEMUIOVector *read_qiov = NULL;
> +
> +    qemu_iovec_init_external(&qiov, iov, iov_cnt);
> +    if (!bdrv_qiov_is_aligned(s->blk->conf.bs, &qiov)) {
> +        /* Redirect I/O to aligned bounce buffer */
> +        bounce_buffer = qemu_blockalign(s->blk->conf.bs, qiov.size);
> +        bounce_iov.iov_base = bounce_buffer;
> +        bounce_iov.iov_len = qiov.size;
> +        iov = &bounce_iov;
> +        iov_cnt = 1;
> +
> +        if (read) {
> +            /* Need to copy back from bounce buffer on completion */
> +            read_qiov = g_slice_new(QEMUIOVector);
> +            qemu_iovec_init(read_qiov, iov_cnt);
> +            qemu_iovec_concat_iov(read_qiov, iov, iov_cnt, 0, qiov.size);

This would have to be the original iov and iov_cnt, but they are already
overwritten here.

> +        } else {
> +            qemu_iovec_to_buf(&qiov, 0, bounce_buffer, qiov.size);
> +        }
> +    }
>  
>      iocb = ioq_rdwr(&s->ioqueue, read, iov, iov_cnt, offset);
>  
> @@ -143,6 +177,8 @@ static int do_rdwr_cmd(VirtIOBlockDataPlane *s, bool read,
>      VirtIOBlockRequest *req = container_of(iocb, VirtIOBlockRequest, iocb);
>      req->head = head;
>      req->inhdr = inhdr;
> +    req->bounce_buffer = bounce_buffer;
> +    req->read_qiov = read_qiov;
>      return 0;
>  }

Kevin