The Critical Reboot: Applying `pci=realloc` to Rescue Eight GPUs in a Proxmox VM

In the span of a single bash command, an AI assistant executing a remote SSH session triggers the culmination of an intricate debugging odyssey. Message [msg 354] in this opencode session is deceptively simple on its surface — a sudo update-grub && sudo reboot issued to a guest virtual machine. But this command represents the convergence of multiple investigative threads: a deep-dive into Linux PCI resource allocation, a discovery about GRUB configuration precedence, a hardware topology constraint analysis, and a carefully planned fix for a problem that had reduced eight NVIDIA RTX PRO 6000 Blackwell GPUs to only two functional devices.

This article examines this single message in detail, unpacking the reasoning, assumptions, mistakes, and knowledge that led to its execution, and what it reveals about the challenges of virtualizing high-performance GPU workloads.

The Problem: Eight GPUs, Only Two Detected

To understand why this message matters, we must first understand the crisis it was designed to resolve. The user was running a Proxmox VE hypervisor with eight NVIDIA RTX PRO 6000 Blackwell GPUs passed through to a single VM. Each of these GPUs carries a massive 96GB of VRAM, which is exposed to the host via a PCIe BAR2 (Base Address Register) region of 128GB (the VRAM mapping). With eight GPUs, the total BAR2 space required is a staggering 1TB — just for the VRAM mappings, not counting the other BAR regions each GPU needs.

The assistant had previously migrated the VM from the legacy i440FX chipset to the modern Q35 chipset with pcie=1 enabled, a necessary step for proper PCIe topology and potential peer-to-peer (P2P) DMA support. However, this migration triggered a severe BAR allocation failure. The guest firmware (SeaBIOS) could not assign the 128GB BAR2 regions for six of the eight GPUs, resulting in the NVIDIA driver reporting BAR2 is 0M @ 0x0 and refusing to probe those devices. Only two GPUs at PCI addresses 04:00.0 and 08:00.0 were successfully initialized.

The guest kernel itself provided the clue: a dmesg message reading pci_bus 0000:00: Some PCI device resources are unassigned, try booting with pci=realloc. This was the kernel's own diagnostic, suggesting a kernel-level reallocation of PCI resources at boot time.

The First Attempt: A Subtle Failure

The assistant's first attempt to apply this fix was straightforward: edit /etc/default/grub on the guest VM to add pci=realloc to GRUB_CMDLINE_LINUX_DEFAULT, run update-grub, and reboot. This was done in messages [msg 344] through [msg 346]. However, after the reboot, the assistant checked the kernel command line and discovered that pci=realloc was not present (cat /proc/cmdline showed only BOOT_IMAGE=/vmlinuz-6.8.0-100-generic root=UUID=... ro console=tty1 console=ttyS0).

This was a critical failure mode that reveals an important assumption the assistant had made: that editing /etc/default/grub was sufficient. In fact, the guest VM was provisioned from a Ubuntu cloud image, and cloud images include a GRUB override file at /etc/default/grub.d/50-cloudimg-settings.cfg. This file sets GRUB_CMDLINE_LINUX_DEFAULT="console=tty1 console=ttyS0", and because it is sourced after /etc/default/grub, its value completely overrides whatever was set in the main configuration file. The assistant's edit to the main file was silently discarded.

This is a classic configuration pitfall in Ubuntu cloud images, and catching it required understanding the GRUB configuration loading order: /etc/default/grub is sourced first, then each file in /etc/default/grub.d/ in alphabetical order. Later assignments to the same variable override earlier ones. The assistant discovered this in message [msg 352] by reading the override file, and then in message [msg 353] applied the fix to the correct file, adding pci=realloc to the cloud-init override.

The Subject Message: Applying the Corrected Fix

Message [msg 354] is the execution of that corrected fix. The assistant runs:

ssh 10.1.230.175 "sudo update-grub && sudo reboot" 2>&1; echo "Rebooting..."

This is a compound command executed over SSH. The && ensures that the reboot only proceeds if update-grub succeeds — a safety measure that prevents rebooting into an unchanged configuration. The 2>&1 redirects stderr to stdout for logging, and the trailing echo "Rebooting..." provides a user-visible confirmation that the command was dispatched.

The output shows the GRUB configuration being regenerated:

Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/50-cloudimg-settings.cfg'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-6.8.0-100-generic
Found initrd image: /boot/initrd.img-6.8.0-100-generic
Found linux image: /boot/vmlinuz-6.8.0-63-generic
Found initrd image: /boot/initrd.img-6.8.0-63-generic
Warning: os-prober will not be executed to detect other bootable partitions.
Systems on them will not be added to the GRUB boot configuration.
Check GRUB_DI...

The output confirms that both GRUB configuration files were sourced, and the new boot configuration was generated. The presence of two kernel versions (6.8.0-100-generic and 6.8.0-63-generic) indicates the system had accumulated multiple kernel updates, and GRUB will boot the newer one by default. The os-prober warning is standard for cloud images and not a concern.

What `pci=realloc` Actually Does

The pci=realloc kernel parameter is a Linux PCI subsystem option that tells the kernel to attempt reallocation of PCI bridge resources and device BARs that the firmware (BIOS or UEFI) failed to assign. When a system has more PCI devices with large BARs than the firmware's MMIO aperture can accommodate, the firmware may leave some BARs unassigned. The kernel can then use its own resource allocator to redistribute the available address space.

In this specific case, the SeaBIOS firmware had allocated bridge windows totaling approximately 1.5TB of 64-bit MMIO space (from 0x380000000000 to 0x395fffffffff), which should theoretically be enough for the 1TB of BAR2 space across eight GPUs. However, the firmware's allocation algorithm failed to place six of the eight BAR2 regions within their respective bridge windows. The kernel, with pci=realloc, can tear down the firmware's assignments and reallocate them more efficiently.

Assumptions and Risks

The assistant made several assumptions in executing this command:

That pci=realloc would resolve the BAR allocation. This was the most significant assumption. While the kernel's own diagnostic suggested this fix, there was no guarantee. The fundamental problem might be that the SeaBIOS firmware's 64-bit MMIO aperture was simply too small, in which case pci=realloc would have no room to work with either. The assistant had a fallback plan (switching to OVMF firmware with an explicitly configured 1.5TB MMIO aperture), but that would be a more invasive change requiring disk conversion from MBR to GPT.

That the SSH session would survive the reboot. The && sudo reboot command would trigger an immediate shutdown. The SSH connection would drop before the command fully completed. The assistant handled this by using 2>&1 to capture all output before the connection terminated, and the trailing echo would only execute locally if the SSH command itself didn't crash. In practice, the SSH client would likely report Connection closed by remote host before the echo, but the important thing was that the GRUB regeneration completed successfully.

That the VM would come back up. A reboot of a VM with problematic PCI passthrough carries risk. If the BAR allocation failure persisted, the VM might boot with only 2 GPUs again, or worse, the kernel might panic trying to reassign resources. The assistant had no way to guarantee a successful boot — it could only wait and check.

That the cloud-init override was the only override. While the assistant had found and fixed the 50-cloudimg-settings.cfg file, there could theoretically be other GRUB override files in /etc/default/grub.d/ that might interfere. The update-grub output confirmed that only the main file and the cloud-init file were sourced, validating this assumption.

The Thinking Process Visible in the Message

While the message itself contains only a bash command and its output, the reasoning behind it is visible in the preceding messages. The assistant had:

Diagnosed the BAR allocation failure by reading the guest's dmesg output and identifying the can't assign; no space errors for BAR2 on six GPUs.
Heeded the kernel's own advice by noting the try booting with pci=realloc message.
Attempted the fix naively by editing /etc/default/grub, which failed because of the cloud-init override.
Investigated the failure by checking /proc/cmdline post-reboot and discovering the override.
Found and fixed the correct file by reading /etc/default/grub.d/50-cloudimg-settings.cfg and adding pci=realloc to the override.
Applied the corrected fix in this message, with the compound update-grub && reboot ensuring atomicity. This is a textbook example of iterative debugging: form a hypothesis, test it, observe the result, refine the hypothesis, and test again. The mistake (editing the wrong file) was a knowledge gap about Ubuntu cloud image GRUB configuration, but the assistant's systematic approach caught and corrected it within two messages.

Input and Output Knowledge

Input knowledge required to understand this message includes: familiarity with Linux PCI resource allocation (BARs, bridge windows, MMIO apertures), understanding of the GRUB configuration chain and cloud-init overrides, knowledge of the pci=realloc kernel parameter and its effects, awareness of the Q35 chipset's PCIe capabilities versus i440FX, and understanding of the NVIDIA driver's BAR probing behavior.

Output knowledge created by this message includes: confirmation that the GRUB configuration was regenerated successfully, evidence that both configuration files were sourced correctly, and the initiation of a reboot that would either confirm or refute the pci=realloc hypothesis. The subsequent messages (not shown in this article's scope) would reveal whether all eight GPUs were detected after the reboot.

Broader Significance

This message illustrates a fundamental challenge in virtualizing high-performance GPU workloads: the interplay between firmware, kernel, and hypervisor in managing PCIe address space. Modern GPUs with large VRAM capacities require enormous contiguous address ranges in the PCIe MMIO space, and virtualized environments add layers of indirection that can break the already delicate BAR allocation process.

The problem is compounded by the fact that cloud-init images, commonly used for VM provisioning, introduce configuration overrides that can silently defeat kernel parameter changes. This is not a bug but a feature — cloud-init is designed to ensure consistent boot configurations across instances — but it creates a trap for administrators who are unaware of the override mechanism.

The assistant's journey from the initial BAR failure to this reboot command also highlights the value of reading kernel diagnostics carefully. The try booting with pci=realloc message in dmesg was not just a suggestion; it was a precise diagnostic pointing to the exact kernel mechanism designed to handle this situation. Following the kernel's own advice is often the fastest path to a fix.

Conclusion

Message [msg 354] is a single bash command that encapsulates an entire debugging arc. It is the moment of execution after a period of investigation, the application of a lesson learned from a previous failure, and a test of a hypothesis about PCI resource allocation. Whether it succeeds or fails, the message represents a critical juncture in the session: the point at which theory meets practice, and the configuration change is committed to the running system.

The command itself is mundane — sudo update-grub && sudo reboot — but the context transforms it into something significant. It is the product of reading dmesg output, understanding PCI bridge windows, discovering GRUB override precedence, and methodically correcting a mistake. In the world of systems administration and AI-assisted debugging, this is how complex problems get solved: not in a single brilliant insight, but in a series of iterative steps, each building on the knowledge gained from the last.

The Critical Reboot: Applying pci=realloc to Rescue Eight GPUs in a Proxmox VM