PCI: Extend isolated function probing to s390

Like the jailhouse hypervisor, s390's PCI architecture allows passing
isolated PCI functions to a guest OS instance. As of now this is was not
utilized even with multi-function support as the s390 PCI code makes sure
that only virtual PCI busses including a function with devfn 0 are
presented to the PCI subsystem. A subsequent change will remove this
restriction.

Allow probing such functions by replacing the existing check for
jailhouse_paravirt() with a new hypervisor_isolated_pci_functions() helper.

Link: https://lore.kernel.org/r/20220628143100.3228092-5-schnelle@linux.ibm.com
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>

drivers/pci/probe.c

@@ -2668,7 +2668,7 @@ int pci_scan_slot(struct pci_bus *bus, int devfn)
 			 * a hypervisor that passes through individual PCI
 			 * functions.
 			 */
-			if (!jailhouse_paravirt())
+			if (!hypervisor_isolated_pci_functions())
 				break;
 		}
 		fn = next_fn(bus, dev, fn);

include/linux/hypervisor.h

@@ -32,4 +32,12 @@ static inline bool jailhouse_paravirt(void)
 
 #endif /* !CONFIG_X86 */
 
+static inline bool hypervisor_isolated_pci_functions(void)
+{
+	if (IS_ENABLED(CONFIG_S390))
+		return true;
+
+	return jailhouse_paravirt();
+}
+
 #endif /* __LINUX_HYPEVISOR_H */