Am 04.01.21 um 17:45 schrieb Alex Williamson:
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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.k... 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).
In this case the guest shouldn't be able to change the config at all and I have no idea what's going wrong here.
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.
That's a rather bad idea. See our GPUs for example return way more than they actually need.
E.g. a Polaris usually returns 4GiB even when only 2GiB are installed, because 4GiB is just the maximum amount of RAM you can put together with the ASIC on a board.
Some devices even return a mask of all 1 even when they need only 2MiB, resulting in nearly 1TiB of wasted address space with this approach.
Regards, Christian.
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