[PATCH v4 05/11] PCI: liveupdate: Inherit bus numbers during Live Update

David Matlack
2026-04-23T21:23:36Z

Inherit bus numbers from the previous kernel during a Live Update when
one or more PCI devices are being preserved.

During a Live Update, preserved devices must be allowed to continue
performing memory transactions so the kernel cannot change the fabric
topology, including bus numbers, since that would require disabling
and flushing any memory transactions first.

To keep things simple, inherit the secondary and subordinate bus numbers
on all bridges if any PCI devices were preserved (i.e. even bridges
without any downstream endpoints that were preserved). This avoids
accidentally assigning a bridge a new window that overlaps with a
preserved device that is downstream of a different bridge.

If a bridge is enumerated with a broken topology or has no bus numbers
set during a Live Update, refuse to assign it new bus numbers and refuse
to enumerate devices below it. This is a safety measure to prevent
topology conflicts.

Require that CONFIG_CARDBUS is not enabled to enable
CONFIG_PCI_LIVEUPDATE since inheriting bus numbers on PCI-to-CardBus
bridges requires additional work but is not a priority at the moment.

Signed-off-by: David Matlack <dmatlack@google.com>
---
.../admin-guide/kernel-parameters.txt | 6 +++-
drivers/pci/Kconfig | 2 +-
drivers/pci/liveupdate.c | 28 +++++++++++++++++++
drivers/pci/probe.c | 21 +++++++++++---
include/linux/pci.h | 1 +
5 files changed, 52 insertions(+), 6 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index cf3807641d89..f412a4b77fb7 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5156,7 +5156,11 @@ Kernel parameters
explicitly which ones they are.
assign-busses [X86] Always assign all PCI bus
numbers ourselves, overriding
- whatever the firmware may have done.
+ whatever the firmware may have done. Ignored
+ during a Live Update, where the kernel must
+ inherit the PCI topology (including bus numbers)
+ to avoid interrupting ongoing memory
+ transactions of preserved devices.
usepirqmask [X86] Honor the possible IRQ mask stored
in the BIOS $PIR table. This is needed on
some systems with broken BIOSes, notably
diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
index 08398cbe970c..6ef457ff9d08 100644
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -330,7 +330,7 @@ config VGA_ARB_MAX_GPUS

config PCI_LIVEUPDATE
bool "PCI Live Update Support (EXPERIMENTAL)"
- depends on PCI && LIVEUPDATE
+ depends on PCI && LIVEUPDATE && !CARDBUS
help
Enable PCI core support for preserving PCI devices across Live
Update. This, in combination with support in a device's driver,
diff --git a/drivers/pci/liveupdate.c b/drivers/pci/liveupdate.c
index c0a30d16d9b8..cf8cff134a75 100644
--- a/drivers/pci/liveupdate.c
+++ b/drivers/pci/liveupdate.c
@@ -93,6 +93,19 @@
* bound to the correct driver. i.e. The PCI core does not protect against a
* device getting preserved by driver A in the outgoing kernel and then getting
* bound to driver B in the incoming kernel.
+ *
+ * BDF Stability
+ * =============
+ *
+ * The PCI core guarantees that incoming preserved devices can be identified by
+ * the same bus, device, and function numbers as prior to kexec. To accomplish
+ * this, the PCI core always inherits the secondary and subordinate bus numbers
+ * assigned to bridges during enumeration, rather than assigning new ones (the
+ * PCI core assumes that the previous kernel established a sane topology).
+ *
+ * If a misconfigured or unconfigured bridge is encountered during enumeration
+ * while there are incoming preserved devices, it's secondary and subordinate
+ * bus numbers will be cleared and devices below it will not be enumerated.
*/

#define pr_fmt(fmt) "PCI: liveupdate: " fmt
@@ -354,6 +367,21 @@ void pci_liveupdate_setup_device(struct pci_dev *dev)
if (!xa)
return;

+ /*
+ * During a Live Update, preserved devices are allowed to continue
+ * performing memory transactions. The kernel must not change the fabric
+ * topology, including bus numbers, since that would require disabling
+ * and flushing any memory transactions first.
+ *
+ * To keep things simple, inherit the secondary and subordinate bus
+ * numbers on _all_ bridges if _any_ PCI devices were preserved (i.e.
+ * even bridges without any downstream endpoints that were preserved).
+ * This avoids accidentally assigning a bridge a new window that
+ * overlaps with a preserved device that is downstream of a different
+ * bridge.
+ */
+ dev->liveupdate_inherit_buses = true;
+
key = pci_ser_xa_key(pci_domain_nr(dev->bus), pci_dev_id(dev));
dev_ser = xa_load(xa, key);

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 938a28e4a7a0..fa26f4170add 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1374,6 +1374,14 @@ bool pci_ea_fixed_busnrs(struct pci_dev *dev, u8 *sec, u8 *sub)
return true;
}

+static bool pci_should_assign_new_buses(struct pci_dev *dev)
+{
+ if (dev->liveupdate_inherit_buses)
+ return false;
+
+ return pcibios_assign_all_busses();
+}
+
/*
* pci_scan_bridge_extend() - Scan buses behind a bridge
* @bus: Parent bus the bridge is on
@@ -1401,6 +1409,7 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
int max, unsigned int available_buses,
int pass)
{
+ const bool assign_new_buses = pci_should_assign_new_buses(dev);
struct pci_bus *child;
u32 buses;
u16 bctl;
@@ -1453,8 +1462,7 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
goto out;
}

- if ((secondary || subordinate) &&
- !pcibios_assign_all_busses() && !broken) {
+ if ((secondary || subordinate) && !assign_new_buses && !broken) {
unsigned int cmax, buses;

/*
@@ -1496,8 +1504,7 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
* do in the second pass.
*/
if (!pass) {
- if (pcibios_assign_all_busses() || broken)
-
+ if (assign_new_buses || broken)
/*
* Temporarily disable forwarding of the
* configuration cycles on all bridges in
@@ -1511,6 +1518,12 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
goto out;
}

+ if (dev->liveupdate_inherit_buses) {
+ pci_err(dev, "Cannot reconfigure bridge during Live Update!\n");
+ pci_err(dev, "Downstream devices will not be enumerated!\n");
+ goto out;
+ }
+
/* Clear errors */
pci_write_config_word(dev, PCI_STATUS, 0xffff);

diff --git a/include/linux/pci.h b/include/linux/pci.h
index dd6b26ca9462..9a602b322e3c 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -511,6 +511,7 @@ struct pci_dev {
unsigned int rom_bar_overlap:1; /* ROM BAR disable broken */
unsigned int rom_attr_enabled:1; /* Display of ROM attribute enabled? */
unsigned int non_mappable_bars:1; /* BARs can't be mapped to user-space */
+ unsigned int liveupdate_inherit_buses:1; /* Inherit bus numbers due to Live Update */
pci_dev_flags_t dev_flags;
atomic_t enable_cnt; /* pci_enable_device has been called */