xen: pvhvm: allow user to request no emulated device unplug

this allows the user to disable pvhvm and revert to emulated devices
in case of a system misconfiguration (e.g. initramfs with only
emulated drivers in it).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>

Documentation/kernel-parameters.txt

@@ -2631,6 +2631,7 @@ and is between 256 and 4096 characters. It is defined in the file
 			all -- unplug all emulated devices (NICs and IDE disks)
 			ignore -- continue loading the Xen platform PCI driver even
 				if the version check failed
+			never -- do not unplug even if version check succeeds
 
 	xirc2ps_cs=	[NET,PCMCIA]
 			Format:

arch/x86/xen/platform-pci-unplug.c

@@ -72,6 +72,9 @@ void __init xen_unplug_emulated_devices(void)
 {
 	int r;
 
+	/* user explicitly requested no unplug */
+	if (xen_emul_unplug & XEN_UNPLUG_NEVER)
+		return;
 	/* check the version of the xen platform PCI device */
 	r = check_platform_magic();
 	/* If the version matches enable the Xen platform PCI driver.
@@ -127,6 +130,8 @@ static int __init parse_xen_emul_unplug(char *arg)
 			xen_emul_unplug |= XEN_UNPLUG_ALL_NICS;
 		else if (!strncmp(p, "ignore", l))
 			xen_emul_unplug |= XEN_UNPLUG_IGNORE;
+		else if (!strncmp(p, "never", l))
+			xen_emul_unplug |= XEN_UNPLUG_NEVER;
 		else
 			printk(KERN_WARNING "unrecognised option '%s' "
 				 "in parameter 'xen_emul_unplug'\n", p);

include/xen/platform_pci.h

@@ -21,6 +21,7 @@
 #define XEN_UNPLUG_AUX_IDE_DISKS 4
 #define XEN_UNPLUG_ALL 7
 #define XEN_UNPLUG_IGNORE 8
+#define XEN_UNPLUG_NEVER 16
 
 static inline int xen_must_unplug_nics(void) {
 #if (defined(CONFIG_XEN_NETDEV_FRONTEND) || \