When the isolated PCI function probing mechanism is used in conjunction
with ARI or SR-IOV it may not find all available PCI functions. In the
case of ARI the problem is that next_ari_fn() always returns -ENODEV if
dev is NULL and thus if fn 0 is missing the scan stops.

For SR-IOV things are more complex. Here the problem is that the check
for multifunction may fail. One example where this can occur is if the
first passed-through function is a VF with devfn 8. Now in
pci_scan_slot() this means it is fn 0 and thus multifunction doesn't get
set. Since VFs don't get multifunction set via PCI_HEADER_TYPE_MFD it
remains unset and probing stops even if there is a devfn 9.

Now at the moment both of these issues are hidden on s390. The first one
because ARI is detected as disabled as struct pci_bus's self is NULL
even though firmware does enable and use ARI. The second issue is hidden
as a side effect of commit 25f39d3dcb48 ("s390/pci: Ignore RID for
isolated VFs"). This is because VFs are either put on their own virtual
bus if the parent PF is not passed-through to the same instance or VFs
are hotplugged once SR-IOV is enabled on the parent PF and then
pci_scan_single_device() is used.

Still especially the first issue prevents correct detection of ARI and
the second might be a problem for other users of isolated function
probing. Fix both issues by keeping things as simple as possible. If
isolated function probing is enabled simply scan every possible devfn.

Fixes: 189c6c33ff42 ("PCI: Extend isolated function probing to s390")
Link: https://lore.kernel.org/linux-pci/d3f11e8562f589ddb2c1c83e74161bd8948084c3.camel@linux.ibm.com/
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
 drivers/pci/probe.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 0ce98e18b5a876afe72af35a9f4a44d598e8d500..13495b12fbcfae4b890bbd4b2f913742adf6dfed 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2808,16 +2808,18 @@ static int next_ari_fn(struct pci_bus *bus, struct pci_dev *dev, int fn)
 	return next_fn;
 }
 
-static int next_fn(struct pci_bus *bus, struct pci_dev *dev, int fn)
+static int next_fn(struct pci_bus *bus, struct pci_dev *dev, int fn, bool isolated)
 {
-	if (pci_ari_enabled(bus))
-		return next_ari_fn(bus, dev, fn);
+	if (!isolated) {
+		if (pci_ari_enabled(bus))
+			return next_ari_fn(bus, dev, fn);
 
+		/* only multifunction devices may have more functions */
+		if (dev && !dev->multifunction)
+			return -ENODEV;
+	}
 	if (fn >= 7)
 		return -ENODEV;
-	/* only multifunction devices may have more functions */
-	if (dev && !dev->multifunction)
-		return -ENODEV;
 
 	return fn + 1;
 }
@@ -2857,12 +2859,14 @@ static int only_one_child(struct pci_bus *bus)
  */
 int pci_scan_slot(struct pci_bus *bus, int devfn)
 {
+	bool isolated_functions;
 	struct pci_dev *dev;
 	int fn = 0, nr = 0;
 
 	if (only_one_child(bus) && (devfn > 0))
 		return 0; /* Already scanned the entire slot */
 
+	isolated_functions = hypervisor_isolated_pci_functions();
 	do {
 		dev = pci_scan_single_device(bus, devfn + fn);
 		if (dev) {
@@ -2876,10 +2880,10 @@ int pci_scan_slot(struct pci_bus *bus, int devfn)
 			 * a hypervisor that passes through individual PCI
 			 * functions.
 			 */
-			if (!hypervisor_isolated_pci_functions())
+			if (!isolated_functions)
 				break;
 		}
-		fn = next_fn(bus, dev, fn);
+		fn = next_fn(bus, dev, fn, isolated_functions);
 	} while (fn >= 0);
 
 	/* Only one slot has PCIe device */

-- 
2.48.1

On Thu, 2025-10-23 at 17:20 +0200, Niklas Schnelle wrote:
> When the isolated PCI function probing mechanism is used in conjunction
> with ARI or SR-IOV it may not find all available PCI functions. In the
> case of ARI the problem is that next_ari_fn() always returns -ENODEV if
> dev is NULL and thus if fn 0 is missing the scan stops.
> 
> For SR-IOV things are more complex. Here the problem is that the check
> for multifunction may fail. One example where this can occur is if the
> first passed-through function is a VF with devfn 8. Now in
> pci_scan_slot() this means it is fn 0 and thus multifunction doesn't get
> set. Since VFs don't get multifunction set via PCI_HEADER_TYPE_MFD it
> remains unset and probing stops even if there is a devfn 9.
> 
> Now at the moment both of these issues are hidden on s390. The first one
> because ARI is detected as disabled as struct pci_bus's self is NULL
> even though firmware does enable and use ARI. The second issue is hidden
> as a side effect of commit 25f39d3dcb48 ("s390/pci: Ignore RID for
> isolated VFs"). This is because VFs are either put on their own virtual
> bus if the parent PF is not passed-through to the same instance or VFs
> are hotplugged once SR-IOV is enabled on the parent PF and then
> pci_scan_single_device() is used.
> 
> Still especially the first issue prevents correct detection of ARI and
> the second might be a problem for other users of isolated function
> probing. Fix both issues by keeping things as simple as possible. If
> isolated function probing is enabled simply scan every possible devfn.
> 
> Fixes: 189c6c33ff42 ("PCI: Extend isolated function probing to s390")

Since I'm adding a fixes tag, I should also add Cc stable. Added for
next revision.

> Link: https://lore.kernel.org/linux-pci/d3f11e8562f589ddb2c1c83e74161bd8948084c3.camel@linux.ibm.com/
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> ---
>  drivers/pci/probe.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
--- snip ---