diff -r 02003bee3e80 xen/arch/x86/hvm/svm/svm.c
--- a/xen/arch/x86/hvm/svm/svm.c	Thu Jun 25 18:31:10 2009 +0100
+++ b/xen/arch/x86/hvm/svm/svm.c	Fri Jun 26 17:19:31 2009 +0200
@@ -111,10 +111,11 @@ static void svm_cpu_down(void)
     write_efer(read_efer() & ~EFER_SVME);
 }
 
-static enum handler_return long_mode_do_msr_write(struct cpu_user_regs *regs)
+static enum handler_return long_mode_do_msr_write(struct vcpu *v,
+                                                  struct cpu_user_regs *regs)
 {
-    u64 msr_content = (u32)regs->eax | ((u64)regs->edx << 32);
-    u32 ecx = regs->ecx;
+    uint64_t msr_content = (uint32_t)regs->eax | ((uint64_t)regs->edx << 32);
+    uint32_t ecx = regs->ecx;
 
     HVM_DBG_LOG(DBG_LEVEL_0, "msr %x msr_content %"PRIx64,
                 ecx, msr_content);
@@ -124,6 +125,11 @@ static enum handler_return long_mode_do_
     case MSR_EFER:
         if ( hvm_set_efer(msr_content) )
             return HNDL_exception_raised;
+        if (!( msr_content & EFER_LME ))
+            break;
+        svm_enable_intercept_for_msr(v, MSR_IA32_SYSENTER_CS);
+        svm_enable_intercept_for_msr(v, MSR_IA32_SYSENTER_ESP);
+        svm_enable_intercept_for_msr(v, MSR_IA32_SYSENTER_EIP);
         break;
 
     case MSR_IA32_MC4_MISC: /* Threshold register */
@@ -1139,7 +1145,7 @@ static int svm_msr_write_intercept(struc
         if ( wrmsr_viridian_regs(ecx, regs->eax, regs->edx) )
             break;
 
-        switch ( long_mode_do_msr_write(regs) )
+        switch ( long_mode_do_msr_write(v, regs) )
         {
         case HNDL_unhandled:
             wrmsr_hypervisor_regs(ecx, regs->eax, regs->edx);
diff -r 02003bee3e80 xen/arch/x86/hvm/svm/vmcb.c
--- a/xen/arch/x86/hvm/svm/vmcb.c	Thu Jun 25 18:31:10 2009 +0100
+++ b/xen/arch/x86/hvm/svm/vmcb.c	Fri Jun 26 17:19:31 2009 +0200
@@ -78,29 +78,36 @@ struct host_save_area *alloc_host_save_a
     return hsa;
 }
 
-void svm_disable_intercept_for_msr(struct vcpu *v, u32 msr)
+void svm_intercept_msr(struct vcpu *v, uint32_t msr, int enable)
 {
     unsigned long *msr_bitmap = v->arch.hvm_svm.msrpm;
+    unsigned long *msr_bit = NULL;
 
     /*
      * See AMD64 Programmers Manual, Vol 2, Section 15.10 (MSR-Bitmap Address).
      */
     if ( msr <= 0x1fff )
     {
-        __clear_bit(msr*2, msr_bitmap + 0x000/BYTES_PER_LONG); 
-        __clear_bit(msr*2+1, msr_bitmap + 0x000/BYTES_PER_LONG); 
+        msr_bit = msr_bitmap + 0x0000 / BYTES_PER_LONG;
     }
     else if ( (msr >= 0xc0000000) && (msr <= 0xc0001fff) )
     {
         msr &= 0x1fff;
-        __clear_bit(msr*2, msr_bitmap + 0x800/BYTES_PER_LONG);
-        __clear_bit(msr*2+1, msr_bitmap + 0x800/BYTES_PER_LONG);
+        msr_bit = msr_bitmap + 0x0800 / BYTES_PER_LONG;
     } 
     else if ( (msr >= 0xc001000) && (msr <= 0xc0011fff) )
     {
         msr &= 0x1fff;
-        __clear_bit(msr*2, msr_bitmap + 0x1000/BYTES_PER_LONG);
-        __clear_bit(msr*2+1, msr_bitmap + 0x1000/BYTES_PER_LONG);
+        msr_bit = msr_bitmap + 0x1000 / BYTES_PER_LONG;
+    }
+
+    BUG_ON(msr_bit == NULL);
+    if (enable) {
+        __set_bit(msr * 2, msr_bit);
+        __set_bit(msr * 2 + 1, msr_bit);
+    } else {
+        __clear_bit(msr * 2, msr_bit);
+        __clear_bit(msr * 2 + 1, msr_bit);
     }
 }
 
@@ -151,6 +158,14 @@ static int construct_vmcb(struct vcpu *v
     svm_disable_intercept_for_msr(v, MSR_STAR);
     svm_disable_intercept_for_msr(v, MSR_SYSCALL_MASK);
 
+    /* Assume 32bit legacy guest. We intercept MSR_EFER and when guest enables
+     * longmode, we enable intercept for the SYSENTER MSRs below
+     * (needed for sysenter/sysexit emulation).
+     */
+    svm_disable_intercept_for_msr(v, MSR_IA32_SYSENTER_CS);
+    svm_disable_intercept_for_msr(v, MSR_IA32_SYSENTER_ESP);
+    svm_disable_intercept_for_msr(v, MSR_IA32_SYSENTER_EIP);
+
     vmcb->msrpm_base_pa = (u64)virt_to_maddr(arch_svm->msrpm);
     vmcb->iopm_base_pa  = (u64)virt_to_maddr(hvm_io_bitmap);
 
diff -r 02003bee3e80 xen/include/asm-x86/hvm/svm/vmcb.h
--- a/xen/include/asm-x86/hvm/svm/vmcb.h	Thu Jun 25 18:31:10 2009 +0100
+++ b/xen/include/asm-x86/hvm/svm/vmcb.h	Fri Jun 26 17:19:31 2009 +0200
@@ -481,7 +481,9 @@ void svm_destroy_vmcb(struct vcpu *v);
 
 void setup_vmcb_dump(void);
 
-void svm_disable_intercept_for_msr(struct vcpu *v, u32 msr);
+#define svm_disable_intercept_for_msr(v, msr) svm_intercept_msr((v), (msr), 0)
+#define svm_enable_intercept_for_msr(v, msr) svm_intercept_msr((v), (msr), 1)
+void svm_intercept_msr(struct vcpu *v, uint32_t msr, int enable);
 
 #endif /* ASM_X86_HVM_SVM_VMCS_H__ */