x86/xsave: simplify xcomp_bv initialization

This eliminates a number of pointless conditionals: Bits 0 and 1 of
xcomp_bv don't matter anyway, and as long as none of bits 2..62 are
set, setting bit 63 is pointless too.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Noticed while working on XSA-165.

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -912,9 +912,7 @@ int arch_set_info_guest(
         if ( v->arch.xsave_area )
         {
             v->arch.xsave_area->xsave_hdr.xstate_bv = XSTATE_FP_SSE;
-            if ( cpu_has_xsaves || cpu_has_xsavec )
-                v->arch.xsave_area->xsave_hdr.xcomp_bv = XSTATE_FP_SSE |
-                                                         XSTATE_COMPACTION_ENABLED;
+            v->arch.xsave_area->xsave_hdr.xcomp_bv = 0;
         }
     }
     else if ( v->arch.xsave_area )
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -2092,9 +2092,7 @@ static int hvm_load_cpu_ctxt(struct doma
 
         memcpy(v->arch.xsave_area, ctxt.fpu_regs, sizeof(ctxt.fpu_regs));
         xsave_area->xsave_hdr.xstate_bv = XSTATE_FP_SSE;
-        if ( cpu_has_xsaves || cpu_has_xsavec )
-            xsave_area->xsave_hdr.xcomp_bv = XSTATE_FP_SSE |
-                                             XSTATE_COMPACTION_ENABLED;
+        xsave_area->xsave_hdr.xcomp_bv = 0;
     }
     else
         memcpy(v->arch.fpu_ctxt, ctxt.fpu_regs, sizeof(ctxt.fpu_regs));
@@ -5562,9 +5560,7 @@ void hvm_vcpu_reset_state(struct vcpu *v
     if ( v->arch.xsave_area )
     {
         v->arch.xsave_area->xsave_hdr.xstate_bv = XSTATE_FP;
-        if ( cpu_has_xsaves || cpu_has_xsavec )
-            v->arch.xsave_area->xsave_hdr.xcomp_bv = XSTATE_FP |
-                                                     XSTATE_COMPACTION_ENABLED;
+        v->arch.xsave_area->xsave_hdr.xcomp_bv = 0;
     }
 
     v->arch.vgc_flags = VGCF_online;
--- a/xen/arch/x86/i387.c
+++ b/xen/arch/x86/i387.c
@@ -270,11 +270,7 @@ int vcpu_init_fpu(struct vcpu *v)
         return rc;
 
     if ( v->arch.xsave_area )
-    {
         v->arch.fpu_ctxt = &v->arch.xsave_area->fpu_sse;
-        if ( cpu_has_xsaves || cpu_has_xsavec )
-            v->arch.xsave_area->xsave_hdr.xcomp_bv = XSTATE_COMPACTION_ENABLED;
-    }
     else
     {
         v->arch.fpu_ctxt = _xzalloc(sizeof(v->arch.xsave_area->fpu_sse), 16);