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Date:   Tue, 17 Nov 2020 18:38:11 +0100
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Stefan Hajnoczi <stefanha@redhat.com>
Cc:     virtualization@lists.linux-foundation.org,
        Laurent Vivier <lvivier@redhat.com>,
        linux-kernel@vger.kernel.org, Eli Cohen <elic@nvidia.com>,
        Jason Wang <jasowang@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Max Gurtovoy <mgurtovoy@nvidia.com>
Subject: Re: [PATCH RFC 04/12] vdpa: add vdpa simulator for block device
Message-ID: <20201117173811.lw4nwicoykhnn3tt@steredhat>
References: <20201113134712.69744-1-sgarzare@redhat.com>
 <20201113134712.69744-5-sgarzare@redhat.com>
 <20201117111121.GD131917@stefanha-x1.localdomain>
 <20201117141620.ytium7r6xpxi4and@steredhat>
 <20201117164342.GT131917@stefanha-x1.localdomain>
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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