linux-kernel - Re: [PATCH v3 4/4] drm/virtio: Support virtgpu exported resources

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Date:   Mon, 8 Jun 2020 19:36:55 +0900
From:   David Stevens <stevensd@...omium.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Gerd Hoffmann <kraxel@...hat.com>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jason Wang <jasowang@...hat.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        open list <linux-kernel@...r.kernel.org>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>, virtio-dev@...ts.oasis-open.org
Subject: Re: [PATCH v3 4/4] drm/virtio: Support virtgpu exported resources

On Mon, Jun 8, 2020 at 6:43 PM Michael S. Tsirkin <mst@...hat.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