Public bug reported: 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/ ** Affects: linux (Ubuntu) Importance: Undecided Status: Invalid ** Affects: linux (Ubuntu Noble) Importance: Undecided Assignee: Mitchell Augustin (mitchellaugustin) Status: In Progress ** Affects: linux (Ubuntu Oracular) Importance: Undecided Assignee: Mitchell Augustin (mitchellaugustin) Status: In Progress ** Changed in: linux (Ubuntu) Assignee: (unassigned) => Mitchell Augustin (mitchellaugustin) ** Changed in: linux (Ubuntu) Status: New => In Progress -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux in Ubuntu. 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: In Progress Status in linux source package in Oracular: In Progress 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/ To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2097389/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : kernel-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
