__HYPERVISOR_COMPAT_VIRT_START for HVM guest with PV drivers

[Chuck Anderson <chuck.anderson@oracle.com>]

Do the guest virtual addresses passed by a hypervisor call on a 32-bit 
HVM with PV drivers guest to a 64-bit hypervisor need to lie below 
__HYPERVISOR_COMPAT_VIRT_START?  Both HVM and PVM domains have their 
hv_compat_vstart set to __HYPERVISOR_COMPAT_VIRT_START.  The 
hypervisor's compat layer is causing the balloon driver's 
HYPERVISOR_memory_op(XENMEM_decrease_reservation, &reservation) 
hypervisor call to error out in xen/common/compat/memory.c 
compat_memory_op(), compat_handle_ok(), because the PVHVM guest's 
extent_start lies above __HYPERVISOR_COMPAT_VIRT_START.  The following 
patch appears to fix the problem but I want to make sure it is fixing 
the real bug:

diff -up xen-3.4.0/xen/arch/x86/domain.c.orig 
xen-3.4.0/xen/arch/x86/domain.c
--- xen-3.4.0/xen/arch/x86/domain.c.orig        2009-08-18 
13:09:30.000000000 -0700
+++ xen-3.4.0/xen/arch/x86/domain.c     2009-08-18 14:55:08.000000000 -0700
@@ -428,7 +428,12 @@ int arch_domain_create(struct domain *d,
 #endif /* __x86_64__ */

 #ifdef CONFIG_COMPAT
-    HYPERVISOR_COMPAT_VIRT_START(d) = __HYPERVISOR_COMPAT_VIRT_START;
+    if (is_hvm_domain(d)) {
+        HYPERVISOR_COMPAT_VIRT_START(d) = 
__HYPERVISOR_COMPAT_VIRT_START_NONE;
+    }
+    else {
+        HYPERVISOR_COMPAT_VIRT_START(d) = __HYPERVISOR_COMPAT_VIRT_START;
+    }
 #endif

    if ( (rc = paging_domain_init(d)) != 0 )
diff -up xen-3.4.0/xen/include/asm-x86/config.h.orig 
xen-3.4.0/xen/include/asm-x86/config.h
--- xen-3.4.0/xen/include/asm-x86/config.h.orig 2009-08-18 
13:22:16.000000000 -0700
+++ xen-3.4.0/xen/include/asm-x86/config.h      2009-08-18 
14:54:40.000000000 -0700
@@ -228,6 +228,7 @@ extern unsigned int video_mode, video_fl

 #ifndef __ASSEMBLY__

+#define __HYPERVISOR_COMPAT_VIRT_START_NONE 0xFFFFFFFF
 /* This is not a fixed value, just a lower limit. */
 #define __HYPERVISOR_COMPAT_VIRT_START 0xF5800000
 #define HYPERVISOR_COMPAT_VIRT_START(d) ((d)->arch.hv_compat_vstart)

Thanks,
Chuck