This bug is awaiting verification that the linux-nvidia-
tegra/6.8.0-1004.4 kernel in -proposed solves the problem. Please test
the kernel and update this bug with the results. If the problem is
solved, change the tag 'verification-needed-noble-linux-nvidia-tegra' to
'verification-done-noble-linux-nvidia-tegra'. If the problem still
exists, change the tag 'verification-needed-noble-linux-nvidia-tegra' to
'verification-failed-noble-linux-nvidia-tegra'.


If verification is not done by 5 working days from today, this fix will
be dropped from the source code, and this bug will be closed.


See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how
to enable and use -proposed. Thank you!


** Tags added: kernel-spammed-noble-linux-nvidia-tegra-v2 
verification-needed-noble-linux-nvidia-tegra

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https://bugs.launchpad.net/bugs/2097389

Title:
  VM boots slowly with large-BAR GPU Passthrough due to pci/probe.c
  redundancy

Status in linux package in Ubuntu:
  Invalid
Status in linux source package in Noble:
  Fix Committed
Status in linux source package in Oracular:
  Fix Committed

Bug description:
  SRU Justification:

  [ Impact ]

  VM guests that have large-BAR GPUs passed through to them will take 2x
  as long to initialize all device BARs without this patch

  [ Test Plan ]

  I verified that this patch applies cleanly to the Noble kernel
  and resolves the bug on DGX H100 and DGX A100. I observed no regressions.
  This can be verified on any machine with a sufficiently large BAR and the
  capability to pass through to a VM using vfio.

  To verify no regressions, I applied this patch to the guest kernel, then
  rebooted and confirmed that:
  1. The measured PCI initialization time on boot was ~50% of the unmodified 
kernel
  2. Relevant parts of /proc/iomem mappings, the PCI init section of dmesg 
output, and lspci -vv output remained unchanged between the system with the 
unmodified kernel and with the patched kernel
  3. The Nvidia driver still successfully loaded and was shown via nvidia-smi 
after the patch was applied

  [ Fix ]

  Roughly half of the time consuming device configuration options invoked during
  the PCI probe function can be eliminated by rearranging the memory and I/O 
disable/enable
  calls such that they only occur per-device rather than per-BAR. This is what 
the upstream
  patch does, and it results in roughly half the excess initialization time 
being eliminated
  reliably during VM boot.

  [ Where problems could occur ]

  I do not expect any regressions. The only callers of ABIs changed by
  this patch are also adjusted within this patch, and the functional
  change only removes entirely redundant calls to disable/enable PCI
  memory/IO.

  [ Additional Context ]

  Upstream patch: 
https://lore.kernel.org/all/20250111210652.402845-1-alex.william...@redhat.com/
  Upstream bug report: 
https://lore.kernel.org/all/cahta-uyp07fgm6t1ozqkqadsa5jrzo0reneyzgqzub4mdrr...@mail.gmail.com/

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