x86/xstate: fix xcomp_bv initialization

We must not clear the compaction bit when using XSAVES/XRSTORS. And
we need to guarantee that xcomp_bv never has any bits clear which
are set in xstate_bv (which requires partly undoing commit 83ae0bb226
["x86/xsave: simplify xcomp_bv initialization"]). Split initialization
of xcomp_bv from the other FPU/SSE/AVX related state setup in
arch_set_info_guest() and hvm_load_cpu_ctxt().

Reported-by: Harmandeep Kaur <write.harmandeep@xxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- unstable.orig/xen/arch/x86/domain.c	2016-01-27 09:29:50.000000000 +0100
+++ unstable/xen/arch/x86/domain.c	2016-01-27 09:52:37.000000000 +0100
@@ -922,15 +922,10 @@ int arch_set_info_guest(
     {
         memcpy(v->arch.fpu_ctxt, &c.nat->fpu_ctxt, sizeof(c.nat->fpu_ctxt));
         if ( v->arch.xsave_area )
-        {
             v->arch.xsave_area->xsave_hdr.xstate_bv = XSTATE_FP_SSE;
-            v->arch.xsave_area->xsave_hdr.xcomp_bv =
-                cpu_has_xsaves ? XSTATE_COMPACTION_ENABLED : 0;
-        }
     }
     else if ( v->arch.xsave_area )
-        memset(&v->arch.xsave_area->xsave_hdr, 0,
-               sizeof(v->arch.xsave_area->xsave_hdr));
+        v->arch.xsave_area->xsave_hdr.xstate_bv = 0;
     else
     {
         typeof(v->arch.xsave_area->fpu_sse) *fpu_sse = v->arch.fpu_ctxt;
@@ -939,6 +934,14 @@ int arch_set_info_guest(
         fpu_sse->fcw = FCW_DEFAULT;
         fpu_sse->mxcsr = MXCSR_DEFAULT;
     }
+    if ( cpu_has_xsaves )
+    {
+        ASSERT(v->arch.xsave_area);
+        v->arch.xsave_area->xsave_hdr.xcomp_bv = XSTATE_COMPACTION_ENABLED |
+            v->arch.xsave_area->xsave_hdr.xstate_bv;
+    }
+    else if ( v->arch.xsave_area )
+        v->arch.xsave_area->xsave_hdr.xcomp_bv = 0;
 
     if ( !compat )
     {
--- unstable.orig/xen/arch/x86/hvm/hvm.c	2015-12-18 12:22:20.000000000 +0100
+++ unstable/xen/arch/x86/hvm/hvm.c	2016-01-27 09:52:26.000000000 +0100
@@ -2094,11 +2094,17 @@ 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;
-        xsave_area->xsave_hdr.xcomp_bv =
-            cpu_has_xsaves ? XSTATE_COMPACTION_ENABLED : 0;
     }
     else
         memcpy(v->arch.fpu_ctxt, ctxt.fpu_regs, sizeof(ctxt.fpu_regs));
+    if ( cpu_has_xsaves )
+    {
+        ASSERT(v->arch.xsave_area);
+        v->arch.xsave_area->xsave_hdr.xcomp_bv = XSTATE_COMPACTION_ENABLED |
+            v->arch.xsave_area->xsave_hdr.xstate_bv;
+    }
+    else if ( v->arch.xsave_area )
+        v->arch.xsave_area->xsave_hdr.xcomp_bv = 0;
 
     v->arch.user_regs.eax = ctxt.rax;
     v->arch.user_regs.ebx = ctxt.rbx;
@@ -5488,8 +5494,8 @@ void hvm_vcpu_reset_state(struct vcpu *v
     if ( v->arch.xsave_area )
     {
         v->arch.xsave_area->xsave_hdr.xstate_bv = XSTATE_FP;
-        v->arch.xsave_area->xsave_hdr.xcomp_bv =
-            cpu_has_xsaves ? XSTATE_COMPACTION_ENABLED : 0;
+        v->arch.xsave_area->xsave_hdr.xcomp_bv = cpu_has_xsaves
+            ? XSTATE_COMPACTION_ENABLED | XSTATE_FP : 0;
     }
 
     v->arch.vgc_flags = VGCF_online;