Re: [RFC PATCH v5 2/2] virtio-vsock: SOCK_SEQPACKET description

[Stefano Garzarella <sgarzare@redhat.com>]

On Wed, Apr 14, 2021 at 09:04:47AM +0300, Arseny Krasnov wrote:
> On 13.04.2021 22:55, Michael S. Tsirkin wrote:
> > On Tue, Apr 13, 2021 at 05:22:44PM +0300, Arseny Krasnov wrote:
> > > On 13.04.2021 16:10, Michael S. Tsirkin wrote:
> > > > On Tue, Apr 13, 2021 at 03:53:29PM +0300, Arseny Krasnov wrote:
> > > > > This adds description of SOCK_SEQPACKET socket type
> > > > > support for virtio-vsock.
> > > > >
> > > > > Signed-off-by: Arseny Krasnov <arseny.krasnov@kaspersky.com>
> > > > > ---
> > > > >  virtio-vsock.tex | 26 +++++++++++++++++++++-----
> > > > >  1 file changed, 21 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/virtio-vsock.tex b/virtio-vsock.tex
> > > > > index ad57f9d..00e59cc 100644
> > > > > --- a/virtio-vsock.tex
> > > > > +++ b/virtio-vsock.tex
> > > > > @@ -16,7 +16,10 @@ \subsection{Virtqueues}\label{sec:Device Types / Socket Device / Virtqueues}
> > > > >
> > > > >  \subsection{Feature bits}\label{sec:Device Types / Socket Device / Feature bits}
> > > > >
> > > > > -There are currently no feature bits defined for this device.
> > > > > +\begin{description}
> > > > > +\item VIRTIO_VSOCK_F_SEQPACKET (0) SOCK_SEQPACKET socket type is
> > > > > +    supported.
> > > > Does it make sense to only support seqpacket and not stream?
> > > > I am guessing not since seqpacket is more or less
> > > > a superset ...
> > > You mean, this sentence must be "Both SOCK_SEQPACKET and SOCK_STREAM types
> > >
> > > are supported"?
> >
> > No. I am asking whether we want a feature bit for SOCK_STREAM too?
>
> I think  there is no practical sense in SOCK_STREAM bit, because SOCK_SEQPACKET
>
> is stream + message boundaries and potential DGRAM is completely different
>
> thing. Of course i can implement it in my patches and also add it to spec patch, but  i see only
>
> esthetic in this: all three socket types have own feature bits.

I agree that it may make sense to have a bit for SOCK_STREAM. For 
example we may have devices in the future that want to implement only 
DGRAM for simplicity.

I'm just worried about backwards compatibility with current devices 
where we don't have any feature bit.

Should we add a negative feature flag? (e.g. VIRTIO_VSOCK_F_NO_STREAM)
I don't like it much, but I can't think of anything better.

Thanks,
Stefano