Messages in this thread | | | Subject | Re: [RFC PATCH] vdpa: mandate 1.0 device | From | Jason Wang <> | Date | Wed, 21 Apr 2021 16:17:08 +0800 |
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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@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'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)?
> > 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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