Re: [RFC PATCH] vdpa: mandate 1.0 device

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Apr 12, 2021 at 02:35:07PM +0800, Jason Wang wrote:
> 
> 在 2021/4/10 上午12:04, Michael S. Tsirkin 写道:
> > On Fri, Apr 09, 2021 at 12:47:55PM +0800, Jason Wang wrote:
> > > 在 2021/4/8 下午11:59, Michael S. Tsirkin 写道:
> > > > On Thu, Apr 08, 2021 at 04:26:48PM +0800, Jason Wang wrote:
> > > > > This patch mandates 1.0 for vDPA devices. The goal is to have the
> > > > > semantic of normative statement in the virtio spec and eliminate the
> > > > > burden of transitional device for both vDPA bus and vDPA parent.
> > > > > 
> > > > > uAPI seems fine since all the vDPA parent mandates
> > > > > VIRTIO_F_ACCESS_PLATFORM which implies 1.0 devices.
> > > > > 
> > > > > For legacy guests, it can still work since Qemu will mediate when
> > > > > necessary (e.g doing the endian conversion).
> > > > > 
> > > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > > Hmm. If we do this, don't we still have a problem with
> > > > legacy drivers which don't ack 1.0?
> > > 
> > > Yes, but it's not something that is introduced in this commit. The legacy
> > > driver never work ...
> > My point is this neither fixes or prevents this.
> > 
> > So my suggestion is to finally add ioctls along the lines
> > of PROTOCOL_FEATURES of vhost-user.
> > 
> > Then that one can have bits for legacy le, legacy be and modern.
> > 
> > BTW I looked at vhost-user and it does not look like that
> > has a solution for this problem either, right?
> 
> 
> Right.
> 
> 
> > 
> > 
> > > > Note 1.0 affects ring endianness which is not mediated in QEMU
> > > > so QEMU can't pretend to device guest is 1.0.
> > > 
> > > Right, I plan to send patches to do mediation in the Qemu to unbreak legacy
> > > drivers.
> > > 
> > > Thanks
> > I frankly think we'll need PROTOCOL_FEATURES anyway, it's too useful ...
> > so why not teach drivers about it and be done with it? You can't emulate
> > legacy on modern in a cross endian situation because of vring
> > endian-ness ...
> 
> 
> So the problem still. This can only work when the hardware can support
> legacy vring endian-ness.
> 
> Consider:
> 
> 1) the leagcy driver support is non-normative in the spec
> 2) support a transitional device in the kenrel may requires the hardware
> support and a burden of kernel codes
> 
> I'd rather simply drop the legacy driver support


My point is this patch does not drop legacy support. It merely mandates
modern support.

> to have a simple and easy
> abstarction in the kenrel. For legacy driver in the guest, hypervisor is in
> charge of the mediation:
> 
> 1) config space access endian conversion
> 2) using shadow virtqueue to change the endian in the vring
> 
> Thanks

I'd like to avoid shadow virtqueue hacks if at all possible.
Last I checked performance wasn't much better than just emulating
virtio in software.

> 
> > 
> > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > > ---
> > > > >    include/linux/vdpa.h | 6 ++++++
> > > > >    1 file changed, 6 insertions(+)
> > > > > 
> > > > > diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h
> > > > > index 0fefeb976877..cfde4ec999b4 100644
> > > > > --- a/include/linux/vdpa.h
> > > > > +++ b/include/linux/vdpa.h
> > > > > @@ -6,6 +6,7 @@
> > > > >    #include <linux/device.h>
> > > > >    #include <linux/interrupt.h>
> > > > >    #include <linux/vhost_iotlb.h>
> > > > > +#include <uapi/linux/virtio_config.h>
> > > > >    /**
> > > > >     * vDPA callback definition.
> > > > > @@ -317,6 +318,11 @@ static inline int vdpa_set_features(struct vdpa_device *vdev, u64 features)
> > > > >    {
> > > > >            const struct vdpa_config_ops *ops = vdev->config;
> > > > > +        /* Mandating 1.0 to have semantics of normative statements in
> > > > > +         * the spec. */
> > > > > +        if (!(features & BIT_ULL(VIRTIO_F_VERSION_1)))
> > > > > +		return -EINVAL;
> > > > > +
> > > > >    	vdev->features_valid = true;
> > > > >            return ops->set_features(vdev, features);
> > > > >    }
> > > > > -- 
> > > > > 2.25.1