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Date:   Thu, 6 Aug 2020 06:00:21 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v2 19/24] vdpa: make sure set_features in invoked for
 legacy
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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:
> > 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 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.

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