Messages in this thread | | | Subject | Re: [RFC PATCH] vdpa: mandate 1.0 device | From | Jason Wang <> | Date | Mon, 12 Apr 2021 14:35:07 +0800 |
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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 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
> > >>> >>> >>> >>> >>>> --- >>>> 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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