diff -r 343470b5ad6b xen/arch/x86/hvm/svm/svm.c
--- a/xen/arch/x86/hvm/svm/svm.c	Tue May 03 14:05:28 2011 -0500
+++ b/xen/arch/x86/hvm/svm/svm.c	Thu May 05 16:40:41 2011 -0500
@@ -716,7 +716,7 @@
     unsigned int eax, ebx, ecx, edx;
     uint32_t msr_low;
     
-    if ( cpu_has_lwp )
+    if ( xsave_enabled(v) && cpu_has_lwp )
     {
         hvm_cpuid(0x8000001c, &eax, &ebx, &ecx, &edx);
         msr_low = (uint32_t)msr_content;
@@ -729,6 +729,9 @@
         /* CPU might automatically correct reserved bits. So read it back. */
         rdmsrl(MSR_AMD64_LWP_CFG, msr_content);
         v->arch.hvm_svm.guest_lwp_cfg = msr_content;
+
+        /* track nonalzy state if LWP_CFG is non-zero. */
+        v->arch.nonlazy_xstate_dirty = !!(msr_content);
     }
 
     return 0;
diff -r 343470b5ad6b xen/arch/x86/i387.c
--- a/xen/arch/x86/i387.c	Tue May 03 14:05:28 2011 -0500
+++ b/xen/arch/x86/i387.c	Thu May 05 16:40:41 2011 -0500
@@ -98,13 +98,13 @@
 /*      FPU Save Functions     */
 /*******************************/
 /* Save x87 extended state */
-static inline void fpu_xsave(struct vcpu *v, uint64_t mask)
+static inline void fpu_xsave(struct vcpu *v)
 {
     /* XCR0 normally represents what guest OS set. In case of Xen itself,
      * we set all accumulated feature mask before doing save/restore.
      */
     set_xcr0(v->arch.xcr0_accum);
-    xsave(v, mask);
+    xsave(v, v->arch.nonlazy_xstate_dirty ? XSTATE_ALL : XSTATE_LAZY);
     set_xcr0(v->arch.xcr0);    
 }
 
@@ -164,15 +164,15 @@
 void vcpu_restore_fpu_eager(struct vcpu *v)
 {
     ASSERT(!is_idle_vcpu(v));
-
-    /* Avoid recursion */
-    clts();
     
     /* save the nonlazy extended state which is not tracked by CR0.TS bit */
-    if ( xsave_enabled(v) )
+    if ( xsave_enabled(v) && v->arch.nonlazy_xstate_dirty )
+    {
+        /* Avoid recursion */
+        clts();
         fpu_xrstor(v, XSTATE_NONLAZY);
-
-    stts();
+        stts();
+    }
 }
 
 /* 
@@ -219,7 +219,7 @@
     clts();
 
     if ( xsave_enabled(v) )
-        fpu_xsave(v, XSTATE_ALL);
+        fpu_xsave(v);
     else if ( cpu_has_fxsr )
         fpu_fxsave(v);
     else
diff -r 343470b5ad6b xen/include/asm-x86/domain.h
--- a/xen/include/asm-x86/domain.h	Tue May 03 14:05:28 2011 -0500
+++ b/xen/include/asm-x86/domain.h	Thu May 05 16:40:41 2011 -0500
@@ -492,6 +492,9 @@
      * it explicitly enables it via xcr0.
      */
     uint64_t xcr0_accum;
+    /* This variable determines whether nonlazy extended state is dirty and 
+     * needs to be tracked. */
+    bool_t nonlazy_xstate_dirty;
 
     struct paging_vcpu paging;