diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 6a47c5915c..a745acd903 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -2546,7 +2546,8 @@ void asmlinkage svm_vmexit_handler(void)
     regs->rsp = vmcb->rsp;
     regs->rflags = vmcb->rflags;
 
-    hvm_invalidate_regs_fields(regs);
+    hvm_sanitize_regs_fields(
+        regs, !(vmcb_get_efer(vmcb) & EFER_LMA) || !(vmcb->cs.l));
 
     if ( paging_mode_hap(v->domain) )
         v->arch.hvm.guest_cr[3] = v->arch.hvm.hw_cr[3] = vmcb_get_cr3(vmcb);
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 23bdf0d195..319a37b745 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -4033,6 +4033,7 @@ static void undo_nmis_unblocked_by_iret(void)
 void asmlinkage vmx_vmexit_handler(struct cpu_user_regs *regs)
 {
     unsigned long exit_qualification, exit_reason, idtv_info, intr_info = 0;
+    unsigned long cs_ar_bytes = 0;
     unsigned int vector = 0;
     struct vcpu *v = current;
     struct domain *currd = v->domain;
@@ -4041,7 +4042,10 @@ void asmlinkage vmx_vmexit_handler(struct cpu_user_regs *regs)
     __vmread(GUEST_RSP,    &regs->rsp);
     __vmread(GUEST_RFLAGS, &regs->rflags);
 
-    hvm_invalidate_regs_fields(regs);
+    if ( hvm_long_mode_active(v) )
+        __vmread(GUEST_CS_AR_BYTES, &cs_ar_bytes);
+
+    hvm_sanitize_regs_fields(regs, !(cs_ar_bytes & X86_SEG_AR_CS_LM_ACTIVE));
 
     if ( paging_mode_hap(v->domain) )
     {
diff --git a/xen/arch/x86/include/asm/hvm/hvm.h b/xen/arch/x86/include/asm/hvm/hvm.h
index 87a6935d97..595253babe 100644
--- a/xen/arch/x86/include/asm/hvm/hvm.h
+++ b/xen/arch/x86/include/asm/hvm/hvm.h
@@ -569,8 +569,24 @@ static inline unsigned int hvm_get_insn_bytes(struct vcpu *v, uint8_t *buf)
             ? alternative_call(hvm_funcs.get_insn_bytes, v, buf) : 0);
 }
 
-static inline void hvm_invalidate_regs_fields(struct cpu_user_regs *regs)
+static inline void hvm_sanitize_regs_fields(struct cpu_user_regs *regs,
+                                            bool compat)
 {
+    if ( compat )
+    {
+        /* Clear GPR upper halves, to counteract guests playing games. */
+        regs->rbp = (uint32_t)regs->ebp;
+        regs->rbx = (uint32_t)regs->ebx;
+        regs->rax = (uint32_t)regs->eax;
+        regs->rcx = (uint32_t)regs->ecx;
+        regs->rdx = (uint32_t)regs->edx;
+        regs->rsi = (uint32_t)regs->esi;
+        regs->rdi = (uint32_t)regs->edi;
+        regs->rip = (uint32_t)regs->eip;
+        regs->rflags = (uint32_t)regs->eflags;
+        regs->rsp = (uint32_t)regs->esp;
+    }
+
 #ifndef NDEBUG
     regs->error_code = 0xbeef;
     regs->entry_vector = 0xbeef;