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Subject: Re: [RFC PATCH] vdpa: mandate 1.0 device
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>
References: <20210408082648.20145-1-jasowang@redhat.com>
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 <20210412050730-mutt-send-email-mst@kernel.org>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <01918e14-7f7a-abf2-5864-292a32f0233c@redhat.com>
Date:   Mon, 12 Apr 2021 17:23:27 +0800
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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?


>
>> 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