x86: determine MXCSR mask in all cases

For its use(s) by the emulator to be correct in all cases, the filling
of the variable needs to be independent of XSAVE availability. As
there's no suitable function in i387.c to put the logic in, keep it in
xstate_init(), arrange for the function to be called unconditionally,
and pull the logic ahead of all return paths there.

Fixes: 9a4496a35b ("x86emul: support {,V}{LD,ST}MXCSR")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/cpu/common.c

@@ -488,8 +488,7 @@ void identify_cpu(struct cpuinfo_x86 *c)
 
 	/* Now the feature flags better reflect actual CPU features! */
 
-	if ( cpu_has_xsave )
-		xstate_init(c);
+	xstate_init(c);
 
 #ifdef NOISY_CAPS
 	printk(KERN_DEBUG "CPU: After all inits, caps:");

xen/arch/x86/xstate.c

@@ -588,6 +588,18 @@ void xstate_init(struct cpuinfo_x86 *c)
     u32 eax, ebx, ecx, edx;
     u64 feature_mask;
 
+    if ( bsp )
+    {
+        static typeof(current->arch.xsave_area->fpu_sse) __initdata ctxt;
+
+        asm ( "fxsave %0" : "=m" (ctxt) );
+        if ( ctxt.mxcsr_mask )
+            mxcsr_mask = ctxt.mxcsr_mask;
+    }
+
+    if ( !cpu_has_xsave )
+        return;
+
     if ( (bsp && !use_xsave) ||
          boot_cpu_data.cpuid_level < XSTATE_CPUID )
     {
@@ -611,8 +623,6 @@ void xstate_init(struct cpuinfo_x86 *c)
 
     if ( bsp )
     {
-        static typeof(current->arch.xsave_area->fpu_sse) __initdata ctxt;
-
         xfeature_mask = feature_mask;
         /*
          * xsave_cntxt_size is the max size required by enabled features.
@@ -621,10 +631,6 @@ void xstate_init(struct cpuinfo_x86 *c)
         xsave_cntxt_size = _xstate_ctxt_size(feature_mask);
         printk("xstate: size: %#x and states: %#"PRIx64"\n",
                xsave_cntxt_size, xfeature_mask);
-
-        asm ( "fxsave %0" : "=m" (ctxt) );
-        if ( ctxt.mxcsr_mask )
-            mxcsr_mask = ctxt.mxcsr_mask;
     }
     else
     {