Re: [PATCH RFC 04/12] vdpa: add vdpa simulator for block device

Stefan Hajnoczi
2020-11-18T11:24:09Z

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On Tue, Nov 17, 2020 at 06:38:11PM +0100, Stefano Garzarella wrote:
> On Tue, Nov 17, 2020 at 04:43:42PM +0000, Stefan Hajnoczi wrote:
> > On Tue, Nov 17, 2020 at 03:16:20PM +0100, Stefano Garzarella wrote:
> > > On Tue, Nov 17, 2020 at 11:11:21AM +0000, Stefan Hajnoczi wrote:
> > > > On Fri, Nov 13, 2020 at 02:47:04PM +0100, Stefano Garzarella wrote:
> > > > > +static void vdpasim_blk_work(struct work_struct *work)
> > > > > +{
> > > > > + struct vdpasim *vdpasim = container_of(work, struct vdpasim, work);
> > > > > + u8 status = VIRTIO_BLK_S_OK;
> > > > > + int i;
> > > > > +
> > > > > + spin_lock(&vdpasim->lock);
> > > > > +
> > > > > + if (!(vdpasim->status & VIRTIO_CONFIG_S_DRIVER_OK))
> > > > > + goto out;
> > > > > +
> > > > > + for (i = 0; i < VDPASIM_BLK_VQ_NUM; i++) {
> > > > > + struct vdpasim_virtqueue *vq = &vdpasim->vqs[i];
> > > > > +
> > > > > + if (!vq->ready)
> > > > > + continue;
> > > > > +
> > > > > + while (vringh_getdesc_iotlb(&vq->vring, &vq->iov, &vq->iov,
> > > > > + &vq->head, GFP_ATOMIC) > 0) {
> > > > > +
> > > > > + int write;
> > > > > +
> > > > > + vq->iov.i = vq->iov.used - 1;
> > > > > + write = vringh_iov_push_iotlb(&vq->vring, &vq->iov, &status, 1);
> > > > > + if (write <= 0)
> > > > > + break;
> > > >
> > > > We're lucky the guest driver doesn't crash after VIRTIO_BLK_T_GET_ID? :)
> > >
> > > The crash could happen if the simulator doesn't put the string terminator,
> > > but in virtio_blk.c, the serial_show() initialize the buffer putting the
> > > string terminator in the VIRTIO_BLK_ID_BYTES element:
> > >
> > > buf[VIRTIO_BLK_ID_BYTES] = '\0';
> > > err = virtblk_get_id(disk, buf);
> > >
> > > This should prevent the issue, right?
> > >
> > > However in the last patch of this series I implemented VIRTIO_BLK_T_GET_ID
> > > support :-)
> >
> > Windows, BSD, macOS, etc guest drivers aren't necessarily going to
> > terminate or initialize the serial string buffer.
>
> Unfortunately I discovered that VIRTIO_BLK_T_GET_ID is not in the VIRTIO
> specs, so, just for curiosity, I checked the QEMU code and I found this:
>
> case VIRTIO_BLK_T_GET_ID:
> {
> /*
> * NB: per existing s/n string convention the string is
> * terminated by '\0' only when shorter than buffer.
> */
> const char *serial = s->conf.serial ? s->conf.serial : "";
> size_t size = MIN(strlen(serial) + 1,
> MIN(iov_size(in_iov, in_num),
> VIRTIO_BLK_ID_BYTES));
> iov_from_buf(in_iov, in_num, 0, serial, size);
> virtio_blk_req_complete(req, VIRTIO_BLK_S_OK);
> virtio_blk_free_request(req);
> break;
> }
>
> It seems that the device emulation in QEMU expects that the driver
> terminates the serial string buffer.
>
> Do you know why VIRTIO_BLK_T_GET_ID is not in the specs?
> Should we add it?

It's about to be merged into the VIRTIO spec:
https://github.com/oasis-tcs/virtio-spec/issues/63

Stefan

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