Re: [RFC PATCH] vdpa: mandate 1.0 device

[Jason Wang <jasowang@redhat.com>]

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