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

Stefano Garzarella
2020-11-17T17:38:22Z

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?

Thanks,
Stefano

>
>Anyway, the later patch that implements VIRTIO_BLK_T_GET_ID solves this
>issue! Thanks.

>
>Stefan