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Subject: Re: [PATCH v2 19/24] vdpa: make sure set_features in invoked for
 legacy
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <cc5cb366-be79-908d-edc6-4aebb488cc59@redhat.com>
Date:   Thu, 6 Aug 2020 15:27:38 +0800
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On 2020/8/6 下午1:53, Michael S. Tsirkin wrote:
> On Thu, Aug 06, 2020 at 11:23:05AM +0800, Jason Wang wrote:
>> On 2020/8/5 下午7:40, Michael S. Tsirkin wrote:
>>> On Wed, Aug 05, 2020 at 02:14:07PM +0800, Jason Wang wrote:
>>>> On 2020/8/4 上午5:00, Michael S. Tsirkin wrote:
>>>>> Some legacy guests just assume features are 0 after reset.
>>>>> We detect that config space is accessed before features are
>>>>> set and set features to 0 automatically.
>>>>> Note: some legacy guests might not even access config space, if this is
>>>>> reported in the field we might need to catch a kick to handle these.
>>>> I wonder whether it's easier to just support modern device?
>>>>
>>>> Thanks
>>> Well hardware vendors are I think interested in supporting legacy
>>> guests. Limiting vdpa to modern only would make it uncompetitive.
>>
>> My understanding is that, IOMMU_PLATFORM is mandatory for hardware vDPA to
>> work.
> Hmm I don't really see why. Assume host maps guest memory properly,
> VM does not have an IOMMU, legacy guest can just work.


Yes, guest may not set IOMMU_PLATFORM.


>
> Care explaining what's wrong with this picture?


The problem is virtio_vdpa, without IOMMU_PLATFORM it uses PA which can 
not work if IOMMU is enabled.

Thanks


>
>
>> So it can only work for modern device ...
>>
>> Thanks
>>
>>
>>>
>>>