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

[David Matlack <dmatlack@google.com>]

On 2026-04-27 08:40 PM, David Matlack wrote:
> On 2026-04-27 11:47 AM, Jacob Pan wrote:
> > On Thu, 23 Apr 2026 21:23:09 +0000
> > David Matlack <dmatlack@google.com> wrote:
> 
> > > 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>
> 
> > > +	/*
> > > +	 * 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;
> > > +
> > This flag never gets cleared after the incoming kernel boot up, what if
> > the user does a manual rescan via sysfs? i.e.
> > # echo 1 > /sys/bus/pci/rescan
> > pcibios_assign_all_busses() will never gets called for this device, and
> > may hit this
> > 	if (dev->liveupdate_inherit_buses) {
> > 		pci_err(dev, "Cannot reconfigure bridge during
> > 		Live Update!\n");
> > 
> > So, maybe clear it in pci_liveupdate_finish()?
> 
> I think we can allo wa rescan to assign new bus numbers once all devices
> go through pci_liveupdate_finish() by clearing
> dev->liveupdate_inherit_buses on all devices in pci_flb_finish(). We
> would need to hold pci_rescan_remove_lock to avoid this racing with such
> a rescan.
> 
> Now that you bring up /sys/bus/pci/rescan... I think we also need to
> set dev->liveupdate_inherit_buses in the outgoing kernel, to avoid bus
> numbers changing on outgoing preserved devices. pci_flb_preserve()
> should take pci_rescan_remove_lock and set dev->liveupdate_inherit_buses
> on all devices, and pci_flb_unpreserve() should do the opposite.
> 
> If we did all then then /sys/bus/pci/rescan can work like normal as long
> as no devices are preserved (incoming or outgoing). If any devices are
> preserved then dev->liveupdate_inherit_buses gets set to prevent bus
> numbers from changing during a possible rescan.

Something like this? This is a diff applied on top of this commit.

diff --git a/drivers/pci/liveupdate.c b/drivers/pci/liveupdate.c
index fead478e8a04..b1b0a5b1a5df 100644
--- a/drivers/pci/liveupdate.c
+++ b/drivers/pci/liveupdate.c
@@ -120,6 +120,20 @@
 
 #include "pci.h"
 
+/*
+ * 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 are 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.
+ */
+static atomic_t inherit_buses;
+
 struct pci_flb_outgoing {
 	/* The pci_ser struct to be passed to the next kernel */
 	struct pci_ser *ser;
@@ -141,6 +155,29 @@ static unsigned long pci_ser_xa_key(unsigned long domain, unsigned long bdf)
 	return domain << 16 | bdf;
 }
 
+bool pci_liveupdate_inherit_buses(void)
+{
+	return atomic_read(&inherit_buses);
+}
+
+static void pci_set_liveupdate_inherit_buses(bool enable)
+{
+	/* Ensure updates to inherit_buses do not race with rescans */
+	pci_lock_rescan_remove();
+
+	/*
+	 * Increment/decrement instead of setting directly to true/false so that
+	 * pci_liveupdate_inherit_buses() returns true if any device is outgoing
+	 * preserved or incoming preserved.
+	 */
+	if (enable)
+		atomic_inc(&inherit_buses);
+	else
+		atomic_dec(&inherit_buses);
+
+	pci_unlock_rescan_remove();
+}
+
 static int pci_flb_preserve(struct liveupdate_flb_op_args *args)
 {
 	struct pci_flb_outgoing *outgoing;
@@ -180,6 +217,8 @@ static int pci_flb_preserve(struct liveupdate_flb_op_args *args)
 
 	args->obj = outgoing;
 	args->data = virt_to_phys(outgoing->ser);
+
+	pci_set_liveupdate_inherit_buses(true);
 	return 0;
 }
 
@@ -187,6 +226,8 @@ static void pci_flb_unpreserve(struct liveupdate_flb_op_args *args)
 {
 	struct pci_flb_outgoing *outgoing = args->obj;
 
+	pci_set_liveupdate_inherit_buses(false);
+
 	pr_debug("Unpreserving struct pci_ser\n");
 	WARN_ON_ONCE(outgoing->ser->nr_devices);
 	kho_unpreserve_free(outgoing->ser);
@@ -223,6 +264,8 @@ static int pci_flb_retrieve(struct liveupdate_flb_op_args *args)
 	}
 
 	args->obj = incoming;
+
+	pci_set_liveupdate_inherit_buses(true);
 	return 0;
 }
 
@@ -230,6 +273,8 @@ static void pci_flb_finish(struct liveupdate_flb_op_args *args)
 {
 	struct pci_flb_incoming *incoming = args->obj;
 
+	pci_set_liveupdate_inherit_buses(false);
+
 	xa_destroy(&incoming->xa);
 	kho_restore_free(incoming->ser);
 	kfree(incoming);
@@ -385,21 +430,6 @@ 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/pci.h b/drivers/pci/pci.h
index 09bab39738d7..abd8379b99cf 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -1442,6 +1442,7 @@ static inline int pci_msix_write_tph_tag(struct pci_dev *pdev, unsigned int inde
 #ifdef CONFIG_PCI_LIVEUPDATE
 void pci_liveupdate_setup_device(struct pci_dev *dev);
 void pci_liveupdate_cleanup_device(struct pci_dev *dev);
+bool pci_liveupdate_inherit_buses(void);
 #else
 static inline void pci_liveupdate_setup_device(struct pci_dev *dev)
 {
@@ -1450,6 +1451,11 @@ static inline void pci_liveupdate_setup_device(struct pci_dev *dev)
 static inline void pci_liveupdate_cleanup_device(struct pci_dev *dev)
 {
 }
+
+static inline bool pci_liveupdate_inherit_buses(void)
+{
+	return false;
+}
 #endif
 
 #endif /* DRIVERS_PCI_H */
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index fa26f4170add..f94fa1fc76cc 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1374,9 +1374,9 @@ 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)
+static bool pci_should_assign_new_buses(void)
 {
-	if (dev->liveupdate_inherit_buses)
+	if (pci_liveupdate_inherit_buses())
 		return false;
 
 	return pcibios_assign_all_busses();
@@ -1409,7 +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);
+	const bool assign_new_buses = pci_should_assign_new_buses();
 	struct pci_bus *child;
 	u32 buses;
 	u16 bctl;
@@ -1518,7 +1518,7 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
 			goto out;
 		}
 
-		if (dev->liveupdate_inherit_buses) {
+		if (pci_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;
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 9a602b322e3c..dd6b26ca9462 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -511,7 +511,6 @@ 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 */