From a371f519daeaf8d8a5462bd9d8cc17061a59f0de Mon Sep 17 00:00:00 2001
From: "Justin T. Gibbs" <gibbs@FreeBSD.org>
Date: Sun, 13 Oct 2013 02:34:20 +0000
Subject: [PATCH] Allow FreeBSD to be booted from CDROM media on XenServer 6.2
 and prior releases.
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Submitted by:	Roger Pau Monné
Sponsored by:	Citrix Systems R&D
Reviewed by:	gibbs
Approved by:	re (gjb)

sys/dev/xen/blkfront/blkfront.c:
	On XenServer versions up to an including 6.2, paravirtualized
	CDROM support is broken.  When running in an HVM domain,
	ignore paravirtualized instances of CDROM media, and instead
	rely on native drivers attaching to emulated hardware.  This
	functions correctly on all currently known Xen based
	platforms.
---
 sys/dev/xen/blkfront/blkfront.c | 38 ++++++++++++++++++++++++++++-----
 1 file changed, 33 insertions(+), 5 deletions(-)

diff --git a/sys/dev/xen/blkfront/blkfront.c b/sys/dev/xen/blkfront/blkfront.c
index feb79a1eb86..92b5f354281 100644
--- a/sys/dev/xen/blkfront/blkfront.c
+++ b/sys/dev/xen/blkfront/blkfront.c
@@ -1381,14 +1381,42 @@ xbd_closing(device_t dev)
 static int
 xbd_probe(device_t dev)
 {
+	if (strcmp(xenbus_get_type(dev), "vbd") != 0)
+		return (ENXIO);
 
-	if (!strcmp(xenbus_get_type(dev), "vbd")) {
-		device_set_desc(dev, "Virtual Block Device");
-		device_quiet(dev);
-		return (0);
+	if (xen_hvm_domain()) {
+		int error;
+		char *type;
+
+		/*
+		 * When running in an HVM domain, IDE disk emulation is
+		 * disabled early in boot so that native drivers will
+		 * not see emulated hardware.  However, CDROM device
+		 * emulation cannot be disabled.
+		 *
+		 * Through use of FreeBSD's vm_guest and xen_hvm_domain()
+		 * APIs, we could modify the native CDROM driver to fail its
+		 * probe when running under Xen.  Unfortunatlely, the PV
+		 * CDROM support in XenServer (up through at least version
+		 * 6.2) isn't functional, so we instead rely on the emulated
+		 * CDROM instance, and fail to attach the PV one here in
+		 * the blkfront driver.
+		 */
+		error = xs_read(XST_NIL, xenbus_get_node(dev),
+		    "device-type", NULL, (void **) &type);
+		if (error)
+			return (ENXIO);
+
+		if (strncmp(type, "cdrom", 5) == 0) {
+			free(type, M_XENSTORE);
+			return (ENXIO);
+		}
+		free(type, M_XENSTORE);
 	}
 
-	return (ENXIO);
+	device_set_desc(dev, "Virtual Block Device");
+	device_quiet(dev);
+	return (0);
 }
 
 /*