? 2021/4/12 ??5:23, Jason Wang ??:>
> ? 2021/4/12 ??5:09, Michael S. Tsirkin ??:
>> On Mon, Apr 12, 2021 at 02:35:07PM +0800, Jason Wang wrote:
>>> ? 2021/4/10 ??12:04, Michael S. Tsirkin ??:
>>>> On Fri, Apr 09, 2021 at 12:47:55PM +0800, Jason Wang wrote:
>>>>> ? 2021/4/8 ??11:59, Michael S. Tsirkin ??:
>>>>>> 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 at
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
>
>
>>
>>> 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
>