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Subject: Re: [RFC PATCH] vdpa: mandate 1.0 device
From:   Jason Wang <jasowang@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, parav@nvidia.com, elic@nvidia.com,
        "Zhu, Lingshan" <lingshan.zhu@intel.com>
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Message-ID: <d5632a4d-4d0b-b08d-06f9-c56f16734607@redhat.com>
Date:   Wed, 21 Apr 2021 15:41:36 +0800
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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


>
>
>>
>>> 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
>