Re: Couple of issues with amdgpu on my WX4100

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 4 Jan 2021 12:34:34 +0100
Christian König <christian.koenig@amd.com> wrote:

> Hi Maxim,
> 
> I can't help with the display related stuff. Probably best approach to 
> get this fixes would be to open up a bug tracker for this on FDO.
> 
> But I'm the one who implemented the resizeable BAR support and your 
> analysis of the problem sounds about correct to me.
> 
> The reason why this works on Linux is most likely because we restore the 
> BAR size on resume (and maybe during initial boot as well).
> 
> See this patch for reference:
> 
> commit d3252ace0bc652a1a244455556b6a549f969bf99
> Author: Christian König <ckoenig.leichtzumerken@gmail.com>
> Date:   Fri Jun 29 19:54:55 2018 -0500
> 
>      PCI: Restore resized BAR state on resume
> 
>      Resize BARs after resume to the expected size again.
> 
>      BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=199959
>      Fixes: d6895ad39f3b ("drm/amdgpu: resize VRAM BAR for CPU access v6")
>      Fixes: 276b738deb5b ("PCI: Add resizable BAR infrastructure")
>      Signed-off-by: Christian König <christian.koenig@amd.com>
>      Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
>      CC: stable@vger.kernel.org      # v4.15+
> 
> 
> It should be trivial to add this to the reset module as well. Most 
> likely even completely vendor independent since I'm not sure what a bus 
> reset will do to this configuration and restoring it all the time should 
> be the most defensive approach.

Hmm, this should already be used by the bus/slot reset path:

pci_bus_restore_locked()/pci_slot_restore_locked()
 pci_dev_restore()
  pci_restore_state()
   pci_restore_rebar_state()

VFIO support for resizeable BARs has been on my todo list, but I don't
have access to any systems that have both a capable device and >4G
decoding enabled in the BIOS.  If we have a consistent view of the BAR
size after the BARs are expanded, I'm not sure why it doesn't just
work.  FWIW, QEMU currently hides the REBAR capability to the guest
because the kernel driver doesn't support emulation through config
space (ie. it's read-only, which the spec doesn't support).

AIUI, resource allocation can fail when enabling REBAR support, which
is a problem if the failure occurs on the host but not the guest since
we have no means via the hardware protocol to expose such a condition.
Therefore the model I was considering for vfio-pci would be to simply
pre-enable REBAR at the max size.  It might be sufficiently safe to
test BAR expansion on initialization and then allow user control, but
I'm concerned that resource availability could change while already in
use by the user.  Thanks,

Alex

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