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

[Stefan Hajnoczi <stefanha@redhat.com>]

On Fri, Jan 11, 2013 at 01:39:40PM +0100, Kevin Wolf wrote:
> 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 <stefanha@redhat.com>
> > ---
> >  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.

Good point.  What's really weird is that I tested this with a Windows
guest - probably the buffer cache fooled me so I didn't notice read was
completely broken.

Stefan