Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 6 Aug 2020 01:53:08 -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
Message-ID: <20200806015112-mutt-send-email-mst@kernel.org>
References: <20200803205814.540410-1-mst@redhat.com>
 <20200803205814.540410-20-mst@redhat.com>
 <9e47d227-f220-4651-dcb9-7a11f059a715@redhat.com>
 <20200805073929-mutt-send-email-mst@kernel.org>
 <bd915b30-0604-da1b-343f-e228bce4d1d8@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <bd915b30-0604-da1b-343f-e228bce4d1d8@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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
> 
> 
> > 
> > 
> >