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Date:   Wed, 21 Apr 2021 04:03:24 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, parav@nvidia.com, elic@nvidia.com,
        "Zhu, Lingshan" <lingshan.zhu@intel.com>
Subject: Re: [RFC PATCH] vdpa: mandate 1.0 device
Message-ID: <20210421035331-mutt-send-email-mst@kernel.org>
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On Wed, Apr 21, 2021 at 03:41:36PM +0800, Jason Wang wrote:
> 
> 在 2021/4/12 下午5:23, Jason Wang 写道:
> > 
> > 在 2021/4/12 下午5:09, Michael S. Tsirkin 写道:
> > > 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.
> > 
> > 
> > I am not sure I get here. This patch fails the set_feature if VERSION_1
> > is not negotiated. This means:
> > 
> > 1) vDPA presents a modern device instead of transitonal device
> > 2) legacy driver can't be probed
> > 
> > What I'm missing?
> 
> 
> Hi Michael:
> 
> Do you agree to find the way to present modern device? We need a conclusion
> to make the netlink API work to move forward.
> 
> Thanks

I think we need a way to support legacy with no data path overhead. qemu
setting VERSION_1 for a legacy guest affects the ring format so it does
not really work. This seems to rule out emulating config space entirely
in userspace.

So I think we should add an ioctl along the lines of
protocol features. Then I think we can reserve feature bits
for config space format: legacy LE, legacy BE, modern.

Querying the feature bits will provide us with info about
what does the device support. Acking them will tell device
what does guest need.


> 
> > 
> > 
> > > 
> > > > 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.
> > 
> > 
> > I think the legacy driver support is just a nice to have. Or do you see
> > any value to that? I guess for mellanox and intel, only modern device is
> > supported in the hardware.
> > 
> > Thanks
> > 
> > 
> > > 
> > > > > 
> > > > > > > 
> > > > > > > 
> > > > > > > 
> > > > > > > > ---
> > > > > > > >     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
> >