On Fri, Sep 03, 2021 at 03:57:24AM -0400, Michael S. Tsirkin wrote:
On Fri, Sep 03, 2021 at 09:22:24AM +0200, Stefano Garzarella wrote:
On Thu, Sep 02, 2021 at 05:08:01PM -0700, Jiang Wang . wrote:
> On Thu, Sep 2, 2021 at 7:07 AM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
> > On Thu, Jun 10, 2021 at 06:12:03PM +0000, Jiang Wang wrote:
[...]
> > >
> > > -There are currently no feature bits defined for this device.
> > > +\begin{description}
> > > +\item[VIRTIO_VSOCK_F_STREAM (0)] Device has support for stream socket type.
> > > +\end{description}
> > > +
> > > +\begin{description}
> > > +\item[VIRTIO_VSOCK_F_DGRAM (2)] Device has support for datagram socket type.
> >
> > Is this really bit 2 or did you mean bit 1 (value 0x2)?
> >
> I left bit 1 for SEQPACKET feature bit. That will probably merge
> before this patch.
Yep, SEQPACKET implementation is also merged in the linux kernel using the
feature bit 1 (0x2), bit 0 (0x1) was left free for stream.
>
> > What happens to the virtqueue layout when VIRTIO_VSOCK_F_DGRAM is
> > present and VIRTIO_VSOCK_F_STREAM is absent? The virtqueue section above
> > implies that VIRTIO_VSOCK_F_STREAM is always present.
> >
> yeah, good question. I think then it means the first two queues will be used
> for dgram?
>
> > > +\end{description}
> > > +
> > > +If no feature bits are defined, assume device only supports stream socket type.
> >
> > It's cleaner to define VIRTIO_VSOCK_F_NO_STREAM (0) instead. When the
> > bit is set the stream socket type is not available and the stream_rx/tx
> > virtqueues are absent.
> >
> > This way it's not necessary to define special behavior depending on
> > certain combinations of feature bits.
> >
> Agree. I went back and read old emails again and found I missed the
> negative bit part. So repeating the previous question, if
> VIRTIO_VSOCK_F_NO_STREAM and VIRTIO_VSOCK_F_DGRAM
> present, then we will only have 3 queues and the first two are for dgram, right?
>
> Also, I am wondering what if an implementation only sets
> VIRTIO_VSOCK_F_NO_STREAM bit, but somehow forgot (or for whatever
> reason) to set VIRTIO_VSOCK_F_DGRAM bit? Does that mean there will
> be no virtqueues? The implementation is a mistake? Because it will not
> do anything.
> Do we need to explicitly add a sentence in the spec to say something like
> "Don't set VIRTIO_VSOCK_F_NO_STREAM alone" etc?
Good point.
IIRC we thought to add F_STREAM to allow devices for example that support
only DGRAM, but we said to consider stream supported if no feature was set
for backward compatibility.
With F_NO_STREAM we can do the same, but actually we could have this strange
case. I don't think it's a big problem, in the end it's a configuration
error. Maybe F_NO_STREMA is clearer since the original device spec supports
streams without any feature bit defined.
Stefano
How about that instead of VIRTIO_VSOCK_F_NO_STREAM we do
VIRTIO_VSOCK_F_TYPE /* device supports multiple socket types */
then with VIRTIO_VSOCK_F_TYPE clear we only have stream.
For SEQPACKET it should be okay, since it depends on stream queues, but
DGRAM will have its own queues, so with F_TYPE it seems to me more
difficult to handle the case in which a device supports DGRAM but not
STREAM.
We should also make SEQPACKET depend on this VIRTIO_VSOCK_F_TYPE -
linux guests do not validate that right now but
it's probably not too late to add such a patch to linux
as a bugfix.
Yep, also with F_NO_STREAM we should do a validation, since F_SEQPACKET
depends on !F_NO_STREAM.
I'll take care of this when we decide what flag to use.
Stefano
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