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Date:   Mon, 8 Jun 2020 11:17:18 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     David Stevens <stevensd@chromium.org>
Cc:     Gerd Hoffmann <kraxel@redhat.com>, David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Jason Wang <jasowang@redhat.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        open list <linux-kernel@vger.kernel.org>,
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        "open list:VIRTIO GPU DRIVER" 
        <virtualization@lists.linux-foundation.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>, virtio-dev@lists.oasis-open.org
Subject: Re: [PATCH v3 4/4] drm/virtio: Support virtgpu exported resources
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On Mon, Jun 08, 2020 at 07:36:55PM +0900, David Stevens wrote:
> On Mon, Jun 8, 2020 at 6:43 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Fri, May 15, 2020 at 04:26:15PM +0900, David Stevens wrote:
> > > > > +     if (virtio_has_feature(vgdev->vdev, VIRTIO_GPU_F_RESOURCE_UUID)) {
> > > > > +             vgdev->has_resource_assign_uuid = true;
> > > > > +     }
> > > >
> > > >
> > > > Just a question: this relies on DMA bufs so I assume it is
> > > > not really assumed to work when DMA API is bypassed, right?
> > > > Rather than worry what does it mean, how about just
> > > > disabling  this feature without PLATFORM_DMA for now?
> > >
> > > By PLATFORM_DMA, do you mean CONFIG_DMA_SHARED_BUFFER?
> >
> > Sorry, no. I mean VIRTIO_F_IOMMU_PLATFORM which in the
> > future will be renamed to VIRTIO_F_PLATFORM_ACCESS.
> 
> Shouldn't things work independent of whether or not that feature is
> set? If a virtio driver properly uses the dma_buf APIs (which virtgpu
> seems to), then that should take care of any mapping/synchronization
> related to VIRTIO_F_IOMMU_PLATFORM. If anything, the case where
> VIRTIO_F_IOMMU_PLATFORM isn't set is easier, since then we know that
> the "the device has same access [sic] to memory addresses supplied to
> it as the driver has", according to the specification.
> 
> -David

I don't know much about drm so I can't tell, I was hoping Gerd can
explain.

-- 
MST