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

[Jason Wang]

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
> >
> >
> > >