Re: [Qemu-devel] [PATCH V3 2/3] virtio-blk: fail get_features when both scsi and 1.0 were set

[Jason Wang <jasowang@redhat.com>]

On 07/22/2015 04:58 PM, Cornelia Huck wrote:
> On Wed, 22 Jul 2015 13:59:51 +0800
> Jason Wang <jasowang@redhat.com> wrote:
>
>> SCSI passthrough was no longer supported in virtio 1.0, so this patch
>> fail the get_features() when both 1.0 and scsi is set. And also only
>> advertise VIRTIO_BLK_F_SCSI for legacy virtio-blk device.
>>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>> ---
>>  hw/block/virtio-blk.c | 9 ++++++++-
>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
>> index 4c27974..4716c3e 100644
>> --- a/hw/block/virtio-blk.c
>> +++ b/hw/block/virtio-blk.c
>> @@ -731,7 +731,14 @@ static uint64_t virtio_blk_get_features(VirtIODevice *vdev, uint64_t features,
>>      virtio_add_feature(&features, VIRTIO_BLK_F_GEOMETRY);
>>      virtio_add_feature(&features, VIRTIO_BLK_F_TOPOLOGY);
>>      virtio_add_feature(&features, VIRTIO_BLK_F_BLK_SIZE);
>> -    virtio_add_feature(&features, VIRTIO_BLK_F_SCSI);
>> +    if (__virtio_has_feature(features, VIRTIO_F_VERSION_1)) {
>> +        if (s->conf.scsi) {
>> +            error_setg(errp, "Virtio 1.0 does not support scsi passthrough!");
>> +            return 0;
>> +        }
>> +    } else {
>> +        virtio_add_feature(&features, VIRTIO_BLK_F_SCSI);
>> +    }
>>
>>      if (s->conf.config_wce) {
>>          virtio_add_feature(&features, VIRTIO_BLK_F_CONFIG_WCE);
> Do we advertise F_SCSI even if scsi is not configured in order to keep
> the same bits as before? I'm afraid I don't remember, that thread was
> long :/
>
> I'm asking because I'd like to depend on that bit to decide whether I
> can negotiate revision 1 for ccw and subsequently offer VERSION_1. It
> would be an easy thing to do, and I'd like to avoid mucking around with
> device-specific configuration from the transport.

I don't see much difference. Both of our patches needs to set scsi to
off to have 1.0 device. And I don't see advantages that did it from
transport. If it has, we probably need something similar in virtio-pci.

>
> To illustrate what I'm talking about, my current patchset for virtio-1
> on ccw is here:
>
> git://github.com/cohuck/qemu virtio-1-ccw-2.5
>

Looks like patch "virtio-blk: scsi is legacy only" breaks backward
compatibility because VIRTIO_BLK_F_SCSI was not notified when scsi is off.

Thanks