Re: [RFC PATCH v1 1/3] virtio: introduce virtio_force_modern()

[Halil Pasic]

On Fri, 12 Nov 2021 16:55:10 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

> On Fri, Nov 12 2021, Halil Pasic <pasic@linux.ibm.com> wrote:
> 
> > On Fri, 29 Oct 2021 16:53:25 +0200
> > Cornelia Huck <cohuck@redhat.com> wrote:
> >  
> >> On Fri, Oct 29 2021, Halil Pasic <pasic@linux.ibm.com> wrote:
> >>   
> >> > Legacy vs modern should be detected via transport specific means. We
> >> > can't wait till feature negotiation is done. Let us introduce
> >> > virtio_force_modern() as a means for the transport code to signal
> >> > that the device should operate in modern mode (because a modern driver
> >> > was detected).
> >> >
> >> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> >> > ---
> >> >
> >> > I'm still struggling with how to deal with vhost-user and co. The
> >> > problem is that I'm not very familiar with the life-cycle of, let us
> >> > say, a vhost_user device.
> >> >
> >> > Looks to me like the vhost part might be just an implementation detail,
> >> > and could even become a hot swappable thing.
> >> >
> >> > Another thing is, that vhost processes set_features differently. It
> >> > might or might not be a good idea to change this.
> >> >
> >> > Does anybody know why don't we propagate the features on features_set,
> >> > but under a set of different conditions, one of which is the vhost
> >> > device is started?
> >> > ---
> >> >  hw/virtio/virtio.c         | 12 ++++++++++++
> >> >  include/hw/virtio/virtio.h |  1 +
> >> >  2 files changed, 13 insertions(+)
> >> >
> >> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> >> > index 3a1f6c520c..75aee0e098 100644
> >> > --- a/hw/virtio/virtio.c
> >> > +++ b/hw/virtio/virtio.c
> >> > @@ -3281,6 +3281,18 @@ void virtio_init(VirtIODevice *vdev, const char 
> >> > *name,
> >> >      vdev->use_guest_notifier_mask = true;
> >> >  }
> >> >  
> >> > +void  virtio_force_modern(VirtIODevice *vdev)    
> >> 
> >> <bikeshed> I'm not sure I like that name. We're not actually forcing the
> >> device to be modern, we just set an early indication in the device
> >> before proper feature negotiation has finished. Maybe
> >> virtio_indicate_modern()? </bikeshed>  
> >
> >
> > I don't like virtio_indicate_modern(dev) form object orientation
> > perspective. In an OO language one would write it like
> > dev.virtio_indicate_modern()
> > which would read like the device should indicate modern to somebody.  
> 
> I think that is actually what happens: we indicate that it is a modern
> device to the code making the endianness decisions.
> 

But in an OO school of thought that code belongs to the given
virtio device object and is one of the building blocks that makes the
object what it is. What I'm trying to explain is: that code ain't no
external entity we have to indicate something to.

On the contrary, if we had to indicate 'modern' to the driver, how would
you name that function? Clearly we don't need such functionality, I'm
just trying to make an argument here.

To take a different example, imagine a ccw channel path. We may break the
the channel path, we may indicate to the OS that the channel path is
broken (via CRW), and we may do first break than indicate.


> >
> > In my opinion what happens is that we want to disable the legacy
> > interface if it is exposed by the device, or in other words instruct the
> > device that should act (precisely and exclusively) according to the
> > interface specification of the modern interface.  
> 
> I don't see us disabling anything; the driver has already chosen what
> they want, and we simply need to make sure that all code honours that
> decision.

IMHO a buggy driver could make an attempt at using the legacy interface
at least in case of pci.

My understanding is that the decision of the driver results in an
interaction between the driver and the device, and as a result of that
interaction, the state of the device changes. This function is supposed
to implement that state-change. 

Do we agree that there is a state change? If yes, how would you describe
that state change?

> 
> >
> > Maybe we can find a better name than force_modern, but I don't think
> > indicate_modern is a better name.
> >  
> >>   
> >> > +{
> >> > +    /*
> >> > +     * This takes care of the devices that implement config space access
> >> > +     * in QEMU. For vhost-user and similar we need to make sure the 
> >> > features
> >> > +     * are actually propagated to the device implementing the config 
> >> > space.
> >> > +     *
> >> > +     * A VirtioDeviceClass callback may be a good idea.
> >> > +     */
> >> > +    virtio_set_features(vdev, (1ULL << VIRTIO_F_VERSION_1));    
> >> 
> >> Do we really need/want to do the whole song-and-dance for setting
> >> features, just for setting VERSION_1?   
> >
> > When doing the whole song-and-dance the chance is higher that the
> > information will propagate to every place it needs to reach. For
> > example to the acked_features of vhost_dev. I've just posted a v2 RFC.
> > It should not be hard to see what I mean after examining that RFC.
> >  
> >> Devices may modify some of their
> >> behaviour or features, depending on what features they are called with,  
> >
> > I believe, if this is the case, we want the behavior that corresponds to
> > VERSION_1 set, i.e. 'modern'. So in my understanding this is rather good
> > than bad.
> >  
> >> and we will be calling this one again later with what is likely a
> >> different feature set.   
> >
> > That is true, but the driver is allowed to set the features multiple
> > times, and since transports only support piecemeal access to the
> > features (32 bits at a time), I guess this is biz as usual.  
> 
> Also see my comment in the v2: I'm not sure how well tested that
> actually is.
> 

Will answer there.

> >  
> >>Also, the return code is not checked.
> >>   
> >
> > That is true! It might be a good idea to log an error. Unfortunately I
> > don't think there is anything else we can sanely do.
> >  
> >> Maybe introduce a new function that sets guest_features directly and
> >> errors out if the features are not set in host_features?   
> >
> > See above.
> >  
> >> If we try to
> >> set VERSION_1 here despite the device not offering it, we are in a
> >> pickle anyway, as we should not have gotten here if we did not offer it,
> >> and we really should moan and fail in that case.  
> >
> > I agree about the moan part. I'm not sure what is the best way to
> > 'fail'. Maybe we should continue this discussion in the v2 thread.  
> 
> Yeah, let's continue there, since that code is a bit different.
> 

Nod!

Thanks!
Halil