vm-assist: prepare for discontiguous used bit numbers

Since the a flag will get assigned a value discontiguous to the
existing ones (in order to preserve the low bits, as only those are
currently accessible to 32-bit guests), this requires a little bit of
rework of the VM assist code in general: An architecture specific
VM_ASSIST_VALID definition gets introduced (with an optional compat
mode counterpart), and compilation of the respective code becomes
conditional upon this being defined (ARM doesn't wire these up and
hence doesn't need that code).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Split original patch into two.

--- a/xen/common/compat/kernel.c
+++ b/xen/common/compat/kernel.c
@@ -41,6 +41,11 @@ CHECK_TYPE(domain_handle);
 #define xennmi_callback compat_nmi_callback
 #define xennmi_callback_t compat_nmi_callback_t
 
+#ifdef COMPAT_VM_ASSIST_VALID
+#undef VM_ASSIST_VALID
+#define VM_ASSIST_VALID COMPAT_VM_ASSIST_VALID
+#endif
+
 #define DO(fn) int compat_##fn
 #define COMPAT
 
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -1318,9 +1318,11 @@ long do_vcpu_op(int cmd, unsigned int vc
     return rc;
 }
 
-long vm_assist(struct domain *p, unsigned int cmd, unsigned int type)
+#ifdef VM_ASSIST_VALID
+long vm_assist(struct domain *p, unsigned int cmd, unsigned int type,
+               unsigned long valid)
 {
-    if ( type > MAX_VMASST_TYPE )
+    if ( type >= BITS_PER_LONG || !test_bit(type, &valid) )
         return -EINVAL;
 
     switch ( cmd )
@@ -1335,6 +1337,7 @@ long vm_assist(struct domain *p, unsigne
 
     return -ENOSYS;
 }
+#endif
 
 struct pirq *pirq_get_info(struct domain *d, int pirq)
 {
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -386,10 +386,12 @@ DO(nmi_op)(unsigned int cmd, XEN_GUEST_H
     return rc;
 }
 
+#ifdef VM_ASSIST_VALID
 DO(vm_assist)(unsigned int cmd, unsigned int type)
 {
-    return vm_assist(current->domain, cmd, type);
+    return vm_assist(current->domain, cmd, type, VM_ASSIST_VALID);
 }
+#endif
 
 DO(ni_hypercall)(void)
 {
--- a/xen/include/asm-x86/config.h
+++ b/xen/include/asm-x86/config.h
@@ -343,6 +343,15 @@ extern unsigned long xen_phys_start;
 #define ARG_XLAT_START(v)        \
     (ARG_XLAT_VIRT_START + ((v)->vcpu_id << ARG_XLAT_VA_SHIFT))
 
+#define NATIVE_VM_ASSIST_VALID   ((1UL << VMASST_TYPE_4gb_segments)        | \
+                                  (1UL << VMASST_TYPE_4gb_segments_notify) | \
+                                  (1UL << VMASST_TYPE_writable_pagetables) | \
+                                  (1UL << VMASST_TYPE_pae_extended_cr3)    | \
+                                  (1UL << VMASST_TYPE_m2p_strict))
+#define VM_ASSIST_VALID          NATIVE_VM_ASSIST_VALID
+#define COMPAT_VM_ASSIST_VALID   (NATIVE_VM_ASSIST_VALID & \
+                                  ((1UL << COMPAT_BITS_PER_LONG) - 1))
+
 #define ELFSIZE 64
 
 #define ARCH_CRASH_SAVE_VMCOREINFO
--- a/xen/include/public/xen.h
+++ b/xen/include/public/xen.h
@@ -486,7 +486,9 @@ DEFINE_XEN_GUEST_HANDLE(mmuext_op_t);
 /* x86/PAE guests: support PDPTs above 4GB. */
 #define VMASST_TYPE_pae_extended_cr3     3
 
+#if __XEN_INTERFACE_VERSION__ < 0x00040600
 #define MAX_VMASST_TYPE                  3
+#endif
 
 #ifndef __ASSEMBLY__
 
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -80,7 +80,8 @@ extern void guest_printk(const struct do
     __attribute__ ((format (printf, 2, 3)));
 extern void noreturn panic(const char *format, ...)
     __attribute__ ((format (printf, 1, 2)));
-extern long vm_assist(struct domain *, unsigned int, unsigned int);
+extern long vm_assist(struct domain *, unsigned int cmd, unsigned int type,
+                      unsigned long valid);
 extern int __printk_ratelimit(int ratelimit_ms, int ratelimit_burst);
 extern int printk_ratelimit(void);