x86/vPIC: vpic_elcr_mask() master bit 2 control

Master bit 2 is treated specially: We force it set, but we don't expose
the bit being set to the guest. While right now the read and write
handling can easily use the fixed mask, the restore input checking that
is about to be put in place wants to use the inverted mask to prove that
no bits are unduly set. That will require master bit 2 to be set. Otoh
the read path requires the bit to be clear (the bit can have either
value for the use on the write path). Hence allow use sites control over
that bit.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/hvm/vpic.c

@@ -41,7 +41,7 @@
 #define vpic_lock(v)   spin_lock(__vpic_lock(v))
 #define vpic_unlock(v) spin_unlock(__vpic_lock(v))
 #define vpic_is_locked(v) spin_is_locked(__vpic_lock(v))
-#define vpic_elcr_mask(v) ((v)->is_master ? 0xf8 : 0xde)
+#define vpic_elcr_mask(v, mb2) ((v)->is_master ? 0xf8 | ((mb2) << 2) : 0xde)
 
 /* Return the highest priority found in mask. Return 8 if none. */
 #define VPIC_PRIO_NONE 8
@@ -387,7 +387,7 @@ static int cf_check vpic_intercept_elcr_io(
         if ( dir == IOREQ_WRITE )
         {
             /* Some IRs are always edge trig. Slave IR is always level trig. */
-            data = (*val >> shift) & vpic_elcr_mask(vpic);
+            data = (*val >> shift) & vpic_elcr_mask(vpic, 1);
             if ( vpic->is_master )
                 data |= 1 << 2;
             vpic->elcr = data;
@@ -395,7 +395,7 @@ static int cf_check vpic_intercept_elcr_io(
         else
         {
             /* Reader should not see hardcoded level-triggered slave IR. */
-            data = vpic->elcr & vpic_elcr_mask(vpic);
+            data = vpic->elcr & vpic_elcr_mask(vpic, 0);
             if ( !shift )
                 *val = data;
             else