x86emul: correct vzero{all,upper} for non-64-bit-mode

The registers only accessible in 64-bit mode need to be left alone in
this case.

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/tools/tests/x86_emulator/test_x86_emulator.c
+++ b/tools/tests/x86_emulator/test_x86_emulator.c
@@ -2910,6 +2910,45 @@ int main(int argc, char **argv)
     else
         printf("skipped\n");
 
+#ifdef __x86_64__
+    printf("%-40s", "Testing vzeroupper (compat)...");
+    if ( cpu_has_avx )
+    {
+        decl_insn(vzeroupper);
+
+        ctxt.sp_size = ctxt.addr_size = 32;
+
+        asm volatile ( "vxorps %xmm2, %xmm2, %xmm3\n"
+                       "vcmpeqps %ymm3, %ymm3, %ymm4\n"
+                       "vmovaps %ymm4, %ymm9\n"
+                       put_insn(vzeroupper, "vzeroupper") );
+
+        set_insn(vzeroupper);
+        rc = x86_emulate(&ctxt, &emulops);
+        if ( rc != X86EMUL_OKAY || !check_eip(vzeroupper) )
+            goto fail;
+
+        /* XMM0...XMM7 should have their high parts cleared. */
+        asm ( "vextractf128 $1, %%ymm4, %%xmm0\n\t"
+              "vpmovmskb %%xmm4, %0\n\t"
+              "vpmovmskb %%xmm0, %1" : "=r" (rc), "=r" (i) );
+        if ( rc != 0xffff || i )
+            goto fail;
+
+        /* XMM8...XMM15 should have their high parts preserved. */
+        asm ( "vextractf128 $1, %%ymm9, %%xmm1\n\t"
+              "vpmovmskb %%xmm9, %0\n\t"
+              "vpmovmskb %%xmm1, %1" : "=r" (rc), "=r" (i) );
+        if ( rc != 0xffff || i != 0xffff )
+            goto fail;
+        printf("okay\n");
+
+        ctxt.sp_size = ctxt.addr_size = 64;
+    }
+    else
+        printf("skipped\n");
+#endif
+
 #undef decl_insn
 #undef put_insn
 #undef set_insn
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -6277,6 +6277,45 @@ x86_emulate(
             generate_exception_if(vex.reg != 0xf, EXC_UD);
             host_and_vcpu_must_have(avx);
             get_fpu(X86EMUL_FPU_ymm, &fic);
+
+#ifdef __x86_64__
+            if ( !mode_64bit() )
+            {
+                /*
+                 * Can't use the actual instructions here, as we must not
+                 * touch YMM8...YMM15.
+                 */
+                if ( vex.l )
+                {
+                    /* vpxor %xmmN, %xmmN, %xmmN */
+                    asm volatile ( ".byte 0xc5,0xf9,0xef,0xc0" );
+                    asm volatile ( ".byte 0xc5,0xf1,0xef,0xc9" );
+                    asm volatile ( ".byte 0xc5,0xe9,0xef,0xd2" );
+                    asm volatile ( ".byte 0xc5,0xe1,0xef,0xdb" );
+                    asm volatile ( ".byte 0xc5,0xd9,0xef,0xe4" );
+                    asm volatile ( ".byte 0xc5,0xd1,0xef,0xed" );
+                    asm volatile ( ".byte 0xc5,0xc9,0xef,0xf6" );
+                    asm volatile ( ".byte 0xc5,0xc1,0xef,0xff" );
+                }
+                else
+                {
+                    /* vpor %xmmN, %xmmN, %xmmN */
+                    asm volatile ( ".byte 0xc5,0xf9,0xeb,0xc0" );
+                    asm volatile ( ".byte 0xc5,0xf1,0xeb,0xc9" );
+                    asm volatile ( ".byte 0xc5,0xe9,0xeb,0xd2" );
+                    asm volatile ( ".byte 0xc5,0xe1,0xeb,0xdb" );
+                    asm volatile ( ".byte 0xc5,0xd9,0xeb,0xe4" );
+                    asm volatile ( ".byte 0xc5,0xd1,0xeb,0xed" );
+                    asm volatile ( ".byte 0xc5,0xc9,0xeb,0xf6" );
+                    asm volatile ( ".byte 0xc5,0xc1,0xeb,0xff" );
+                }
+
+                put_fpu(&fic);
+
+                ASSERT(!state->simd_size);
+                break;
+            }
+#endif
         }
         else
         {