linux-kernel - Re: [PATCH 0/2] Improve vfio-pci primary GPU assignment behavior

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Message-ID: <20220608074306.wyav3oerq5crdk6c@sirius.home.kraxel.org>
Date:   Wed, 8 Jun 2022 09:43:06 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie, daniel@...ll.ch,
        kvm@...r.kernel.org, Laszlo Ersek <lersek@...hat.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Improve vfio-pci primary GPU assignment behavior

  Hi,

> But also, this issue isn't something that only affects graphic devices,
> right? AFAIU from [1] and [2], the same issue happens if a PCI device
> has to be bound to vfio-pci but already was bound to a host driver.

Nope.  There is a standard procedure to bind and unbind pci drivers via
sysfs, using /sys/bus/pci/drivers/$name/{bind,unbind}.

> The fact that DRM happens to have some infrastructure to remove devices
> that conflict with an aperture is just a coincidence.

No.  It's a consequence of firmware framebuffers not being linked to the
pci device actually backing them, so some other way is needed to find
and solve conflicts.

> The series [0] mentioned above, adds a sysfb_disable() that disables the
> Generic System Framebuffer logic that is what registers the framebuffer
> devices that are bound to these generic video drivers. On disable, the
> devices registered by sysfb are also unregistered.

As Alex already mentioned this might not have the desired effect on
systems with multiple GPUs (I think even without considering vfio-pci).

> That is, do you want to remove the {vesa,efi,simple}fb and simpledrm
> drivers or is there a need to also remove real fbdev and DRM drivers?

Boot framebuffers are the problem because they are neither visible nor
manageable in /sys/bus/pci.  For real fbdev/drm drivers the standard pci
unbind can be used.

take care,
  Gerd