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References: <20230227173800.2809727-1-robdclark@gmail.com> <44e67d88-fce6-a1c1-79a9-a937e64a32bb@collabora.com>
In-Reply-To: <44e67d88-fce6-a1c1-79a9-a937e64a32bb@collabora.com>
From:   Rob Clark <robdclark@gmail.com>
Date:   Mon, 27 Feb 2023 10:15:34 -0800
Message-ID: <CAF6AEGsT8_o+v0vzGu1nyh6Z82pj8FnGUdMFc0Lq+4OWoSjRBQ@mail.gmail.com>
Subject: Re: [PATCH v3] drm/virtio: Add option to disable KMS support
To:     Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc:     dri-devel@lists.freedesktop.org, Chia-I Wu <olvaffe@gmail.com>,
        Ryan Neph <ryanneph@chromium.org>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        Rob Clark <robdclark@chromium.org>,
        David Airlie <airlied@redhat.com>,
        Gurchetan Singh <gurchetansingh@chromium.org>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@lists.linux-foundation.org>,
        open list <linux-kernel@vger.kernel.org>
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On Mon, Feb 27, 2023 at 9:57 AM Dmitry Osipenko
<dmitry.osipenko@collabora.com> wrote:
>
> On 2/27/23 20:38, Rob Clark wrote:
> ...
> > +     if (IS_ENABLED(CONFIG_DRM_VIRTIO_GPU_KMS)) {
> > +             /* get display info */
> > +             virtio_cread_le(vgdev->vdev, struct virtio_gpu_config,
> > +                             num_scanouts, &num_scanouts);
> > +             vgdev->num_scanouts = min_t(uint32_t, num_scanouts,
> > +                                         VIRTIO_GPU_MAX_SCANOUTS);
> > +             if (!vgdev->num_scanouts) {
> > +                     /*
> > +                      * Having an EDID but no scanouts is non-sensical,
> > +                      * but it is permitted to have no scanouts and no
> > +                      * EDID (in which case DRIVER_MODESET and
> > +                      * DRIVER_ATOMIC are not advertised)
> > +                      */
> > +                     if (vgdev->has_edid) {
> > +                             DRM_ERROR("num_scanouts is zero\n");
> > +                             ret = -EINVAL;
> > +                             goto err_scanouts;
> > +                     }
> > +                     dev->driver_features &= ~(DRIVER_MODESET | DRIVER_ATOMIC);
>
> If it's now configurable by host, why do we need the
> CONFIG_DRM_VIRTIO_GPU_KMS?

Because a kernel config option makes it more obvious that
modeset/atomic ioctls are blocked.  Which makes it more obvious about
where any potential security issues apply and where fixes need to get
backported to.  The config option is the only thing _I_ want,
everything else is just a bonus to help other people's use-cases.

BR,
-R