? 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?
>
>> 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