xen: don't use privcmd_call() from xen_mc_flush()

Using privcmd_call() for a singleton multicall seems to be wrong, as
privcmd_call() is using stac()/clac() to enable hypervisor access to
Linux user space.

Even if currently not a problem (pv domains can't use SMAP while HVM
and PVH domains can't use multicalls) things might change when
PVH dom0 support is added to the kernel.
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

arch/x86/include/asm/xen/hypercall.h

@@ -209,24 +209,37 @@ extern struct { char _entry[32]; } hypercall_page[];
 })
 
 static inline long
-privcmd_call(unsigned call,
-	     unsigned long a1, unsigned long a2,
-	     unsigned long a3, unsigned long a4,
-	     unsigned long a5)
+xen_single_call(unsigned int call,
+		unsigned long a1, unsigned long a2,
+		unsigned long a3, unsigned long a4,
+		unsigned long a5)
 {
 	__HYPERCALL_DECLS;
 	__HYPERCALL_5ARG(a1, a2, a3, a4, a5);
 
-	stac();
 	asm volatile(CALL_NOSPEC
 		     : __HYPERCALL_5PARAM
 		     : [thunk_target] "a" (&hypercall_page[call])
 		     : __HYPERCALL_CLOBBER5);
-	clac();
 
 	return (long)__res;
 }
 
+static inline long
+privcmd_call(unsigned int call,
+	     unsigned long a1, unsigned long a2,
+	     unsigned long a3, unsigned long a4,
+	     unsigned long a5)
+{
+	long res;
+
+	stac();
+	res = xen_single_call(call, a1, a2, a3, a4, a5);
+	clac();
+
+	return res;
+}
+
 static inline int
 HYPERVISOR_set_trap_table(struct trap_info *table)
 {

arch/x86/xen/multicalls.c

@@ -80,9 +80,9 @@ void xen_mc_flush(void)
 		   and just do the call directly. */
 		mc = &b->entries[0];
 
-		mc->result = privcmd_call(mc->op,
-					  mc->args[0], mc->args[1], mc->args[2], 
-					  mc->args[3], mc->args[4]);
+		mc->result = xen_single_call(mc->op, mc->args[0], mc->args[1],
+					     mc->args[2], mc->args[3],
+					     mc->args[4]);
 		ret = mc->result < 0;
 		break;