Public bug reported: Due to an inefficiency in the way older host kernels manage pfnmaps for guest VM memory ranges[0], guests with large-BAR GPUs passed through have a very long (multiple minutes) initialization time when the MMIO window advertised by OVMF is sufficiently sized for the passed-through BARs (i.e., the correct OVMF behavior). However, in the past, users have benefited from fast guest boot times when OVMF advertised an MMIO window that was too small to accommodate the full BAR, since this resulted in the long PCI initialization process being skipped (and retried later, if pci=realloc pci=nocrs were set).
While the root cause is being fully addressed in the upstream kernel[1], the solution relies on huge pfnmap support, which is not expected to be backported into the 6.8 or 6.11 -generic kernels. As a result, the only kernel improvement supported on those kernels is this patch[2], which reduces the extra boot time by about half. Unfortunately, that boot time is still an average of 1-3 minutes longer per-VM-boot than what can be achieved when the host is running a version of OVMF without PlatformDynamicMmioWindow (PDMW) support (introduced in [3]) (as was the case in Jammy's version of OVMF). Since there is no way to force the use of the classic MMIO window size[4] in any version of OVMF after [3], and since we have a use case for such functionality on Noble that would yield significant, recurring compute time savings across all impacted VMs, I propose adding a boolean fw_cfg knob "opt/ovmf/X-PciMmioClassicWindow", which, when TRUE, would bypass the PDMW functionality in [3] and instead use the "classic" MMIO window. When set, users' guest VMs would need to set the `pci=nocrs pci=realloc` kernel options for passed-through GPUs to be usable, but boots would be short again, as they were in Jammy. I am *not* planning to submit this patch to upstream edk2 since this option should not be necessary for any users running newer kernels, once the requisite patches[1] land in the mainline tree. [0]: https://lore.kernel.org/all/cahta-uyp07fgm6t1ozqkqadsa5jrzo0reneyzgqzub4mdrr...@mail.gmail.com/ [1]: https://lore.kernel.org/all/20250205231728.2527186-1-alex.william...@redhat.com/ [2]: https://lore.kernel.org/all/20250111210652.402845-1-alex.william...@redhat.com/ [3]: https://github.com/tianocore/edk2/commit/ecb778d0ac62560aa172786ba19521f27bc3f650 [4]: https://edk2.groups.io/g/devel/topic/109651206?p=Created,,,20,1,0,0 ** Affects: edk2 (Ubuntu) Importance: Undecided Assignee: Mitchell Augustin (mitchellaugustin) Status: Confirmed ** Changed in: edk2 (Ubuntu) Assignee: (unassigned) => Mitchell Augustin (mitchellaugustin) ** Changed in: edk2 (Ubuntu) Status: New => Confirmed -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2101903 Title: UBUNTU: SAUCE: Introduce X-PciMmioClassicWindow option to fw_cfg To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/edk2/+bug/2101903/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
