pci: Save deferred BAR reprogramming state - #7945
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rbradford merged 1 commit intoApr 1, 2026
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OVMF can reprogram PCI BARs while memory space decoding is disabled. Cloud Hypervisor defers the corresponding BAR move in `pending_bar_reprogram` until the PCI command register enables Memory Space again. That deferred state was not part of `PciConfigurationState`. A snapshot taken in that window restored the new BAR values in PCI config space, but lost the pending BAR relocation needed to update the VMM-side BAR mapping. The restore logs show guest MMIO accesses to the reprogrammed BAR addresses `0xc0000000`, `0x100000000`, and `0x100080000` hitting unregistered addresses. The firmware serial output shows OVMF assigning those same BAR addresses during PCI resource allocation, then reaching BDS, finding the mass-storage device, and failing to boot from it. Serialize and restore `pending_bar_reprogram` so deferred BAR moves survive snapshot and restore. Co-authored-by: Thomas Prescher <[email protected]> Co-authored-by: Julian Schindel <[email protected]> On-behalf-of: SAP [email protected] Signed-off-by: Leander Kohler <[email protected]>
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We expect there might be more bugs during early boot (mid-firmware) snapshotting. We will continue debugging and upstream the fixes as they are discovered. |
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Urgh, yeh - snapshot/restore or migration early in the boot is definitely gnarly. Thanks for looking into that!
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This PR serializes and restores
pending_bar_reprogramso pending BAR updates survive snapshot and restore.OVMF can reprogram PCI BARs while memory space decoding is disabled. Cloud Hypervisor defers the corresponding BAR move in
pending_bar_reprogramuntil memory space is enabled again via the PCI command register.That deferred state is not part of
PciConfigurationState. If a snapshot is taken in that window, the restored guest sees the updated BAR values in PCI config space, but the VMM-side BAR mapping is still stale.We observed this during early boot while snapshotting mid-firmware (EDK2). In the firmware debug log, OVMF assigns BARs at
0xc0000000,0x100000000, and0x100080000. After restore, cloud-hypervisor logs repeated guest MMIO accesses to those same addresses as unregistered, for example:The firmware continues into Boot Device Selection, still detects the mass-storage device, but can no longer boot from it:
We have implemented this fix in our fork at cyberus-technology#135.