Re: [PATCH v2 19/24] vdpa: make sure set_features in invoked for legacy

On Thu, Aug 06, 2020 at 03:27:38PM +0800, Jason Wang wrote:

On 2020/8/6 下午1:53, Michael S. Tsirkin wrote:

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On Thu, Aug 06, 2020 at 11:23:05AM +0800, Jason Wang wrote:

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On 2020/8/5 下午7:40, Michael S. Tsirkin wrote:

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On Wed, Aug 05, 2020 at 02:14:07PM +0800, Jason Wang wrote:

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On 2020/8/4 上午5:00, Michael S. Tsirkin wrote:

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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.

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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 that's a virtio_vdpa limitation. In the same way, if a device
does not have an on-device iommu *and* is not behind an iommu,
then vdpa can't bind to it.

But this virtio_vdpa specific hack does not belong in a generic vdpa code.

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So it can only work for modern device ...

Thanks

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