Date:   Tue, 11 May 2021 04:43:13 -0400 (EDT)
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, parav@...dia.com, elic@...dia.com,
        Lingshan Zhu <lingshan.zhu@...el.com>
Subject: Re: [RFC PATCH] vdpa: mandate 1.0 device



----- 原始邮件 -----
> 
> 在 2021/4/21 下午4:03, Michael S. Tsirkin 写道:
> > 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@...hat.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'd rather drop the legacy support in this case. It never work for
> vDPA in the past and virtio-vDPA doesn't even need that. Note that
> ACCESS_PLATFORM is mandated for all the vDPA parents right now which
> implies modern device and LE. I wonder what's the value for supporting
> legacy in this case or do we really encourage vendors to ship card with
> legacy support (e.g endian support in the hardware)?

Hi Michael:

Any thoughts on this approach?

My understanding is that dropping legacy support will simplify a lot of stuffs.

Thanks


> 
> 
> >
> > 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.
> 
> 
> We had VHOST_SET_VRING_ENDIAN but this will complicates both the vDPA
> parent and management. What's more important, legacy behaviour is not
> restrictied by the spec.
> 
> 
> >
> > Querying the feature bits will provide us with info about
> > what does the device support. Acking them will tell device
> > what does guest need.
> 
> 
> I think this can work, but I wonder how much we can gain from such
> complexitiy.
> 
> Thanks
> 
> 
> >
> >
> >
> >
> >
> >>>
> >>>>> 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
> 
> 

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