x86/HVM: never retain emulated insn cache when exiting back to guest

Commit 5fcb26e69e ("x86/HVM: don't retain emulated insn cache when
exiting back to guest") didn't go quite far enough: The insn emulator
may itself decide to return X86EMUL_RETRY (currently for certain
CMPXCHG failures and AVX2 gather insns), in which case we'd also exit
back to guest context. Tie the caching to whether we have an I/O
completion pending, instead of x86_emulate()'s return value.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

xen/arch/x86/hvm/emulate.c

@@ -2266,22 +2266,20 @@ static int _hvm_emulate_one(struct hvm_emulate_ctxt *hvmemul_ctxt,
 
     vio->mmio_retry = 0;
 
-    switch ( rc = x86_emulate(&hvmemul_ctxt->ctxt, ops) )
+    rc = x86_emulate(&hvmemul_ctxt->ctxt, ops);
+    if ( rc == X86EMUL_OKAY && vio->mmio_retry )
+        rc = X86EMUL_RETRY;
+
+    if ( !hvm_vcpu_io_need_completion(vio) )
     {
-    case X86EMUL_OKAY:
-        if ( vio->mmio_retry )
-            rc = X86EMUL_RETRY;
-        /* fall through */
-    default:
         vio->mmio_cache_count = 0;
         vio->mmio_insn_bytes = 0;
-        break;
-
-    case X86EMUL_RETRY:
+    }
+    else
+    {
         BUILD_BUG_ON(sizeof(vio->mmio_insn) < sizeof(hvmemul_ctxt->insn_buf));
         vio->mmio_insn_bytes = hvmemul_ctxt->insn_buf_bytes;
         memcpy(vio->mmio_insn, hvmemul_ctxt->insn_buf, vio->mmio_insn_bytes);
-        break;
     }
 
     if ( hvmemul_ctxt->ctxt.retire.singlestep )