linux-kernel - Re: [PATCH v2 19/24] vdpa: make sure set

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Date:   Thu, 6 Aug 2020 01:53:08 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2 19/24] vdpa: make sure set_features in invoked for
 legacy

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.

Care explaining what's wrong with this picture?


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