Message-ID: <33f03f9e-4af3-3237-00dc-d79570d9022a@collabora.com>
Date:   Tue, 28 Feb 2023 14:23:37 +0300
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Subject: Re: [PATCH] drm/virtio: Add option to disable KMS support
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To:     Javier Martinez Canillas <javierm@redhat.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Rob Clark <robdclark@gmail.com>
Cc:     Rob Clark <robdclark@chromium.org>,
        open list <linux-kernel@vger.kernel.org>,
        dri-devel@lists.freedesktop.org,
        Gurchetan Singh <gurchetansingh@chromium.org>,
        Ryan Neph <ryanneph@chromium.org>,
        David Airlie <airlied@redhat.com>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@lists.linux-foundation.org>
References: <20230224180225.2477641-1-robdclark@gmail.com>
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 <20230228062809.ccyzgnvizh6jidn4@sirius.home.kraxel.org>
 <87a60yywo0.fsf@minerva.mail-host-address-is-not-set>
From:   Dmitry Osipenko <dmitry.osipenko@collabora.com>
In-Reply-To: <87a60yywo0.fsf@minerva.mail-host-address-is-not-set>
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On 2/28/23 12:19, Javier Martinez Canillas wrote:
> Gerd Hoffmann <kraxel@redhat.com> writes:
> 
> Hello Gerd,
> 
>> On Mon, Feb 27, 2023 at 07:40:11AM -0800, Rob Clark wrote:
>>> On Sun, Feb 26, 2023 at 10:38 PM Gerd Hoffmann <kraxel@redhat.com> wrote:
>>>> On Fri, Feb 24, 2023 at 10:02:24AM -0800, Rob Clark wrote:
>>>>> From: Rob Clark <robdclark@chromium.org>
>>>>>
>>>>> Add a build option to disable modesetting support.  This is useful in
>>>>> cases where the guest only needs to use the GPU in a headless mode, or
>>>>> (such as in the CrOS usage) window surfaces are proxied to a host
>>>>> compositor.
>>>> Why make that a compile time option?  There is a config option for the
>>>> number of scanouts (aka virtual displays) a device has.  Just set that
>>>> to zero (and fix the driver to not consider that configuration an
>>>> error).
>>> The goal is to not advertise DRIVER_MODESET (and DRIVER_ATOMIC).. I
>>> guess that could be done based on whether there are any scanouts, but
>>> it would mean making the drm_driver struct non-const.
>> Apparently there is a drm_device->driver_features override,
>> (amdgpu uses that).  The driver could simply drop the DRIVER_MODESET and
>> DRIVER_ATOMIC bits in case no scanout is present instead of throwing an
>> error.
>>
>>> And I think it is legitimate to allow the guest to make this choice,
>>> regardless of what the host decides to expose, since it is about the
>>> ioctl surface area that the guest kernel exposes to guest userspace.
>> I think it is a bad idea to make that a compile time option, I'd suggest
>> a runtime switch instead, for example a module parameter to ask the
>> driver to ignore any scanouts.
>>
> I don't think there's a need for a new module parameter, there's already
> the virtio-gpu 'modeset' module parameter to enable/disable modsetting
> and the global 'nomodeset' kernel cmdline parameter to do it for all DRM
> drivers.
> 
> Currently, many drivers just fail to probe when 'nomodeset' is present,
> but others only disable modsetting but keep the rendering part. In fact,
> most DRM only drivers just ignore the 'nomodeset' parameter.

IIUC, Rob's main point for having a config option is solely for security
reasons. The config option eliminates possibility of accidentally (or
intentionally) enabling KMS from software, which is better to have in
case of shipping a product (Chromebook) on which multiple teams are
working on.

-- 
Best regards,
Dmitry