Re: [PATCH 03/13] x86/HV: Add new hvcall guest address host visibility support

On 7/29/2021 1:06 AM, Dave Hansen wrote:

On 7/28/21 7:52 AM, Tianyu Lan wrote:

quoted

@@ -1986,7 +1988,9 @@ static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
  	int ret;
  
  	/* Nothing to do if memory encryption is not active */
-	if (!mem_encrypt_active())
+	if (hv_is_isolation_supported())
+		return hv_set_mem_enc(addr, numpages, enc);
+	else if (!mem_encrypt_active())
  		return 0;

One more thing.  If you're going to be patching generic code, please
start using feature checks that can get optimized away at runtime.
hv_is_isolation_supported() doesn't look like the world's cheapest
check.  It can't be inlined and costs at least a function call.

Yes, you are right. How about adding a static branch key for the check 
of isolation VM? This may reduce the check cost.