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

From: Jason Wang
Date: Thu Aug 06 2020 - 03:27:52 EST

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?

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
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.


So it can only work for modern device ...