On 7/29/21 6:01 AM, Tianyu Lan wrote:

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

quoted

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.

I don't think you need a static key.

There are basically three choices:
1. Use an existing X86_FEATURE bit.  I think there's already one for
   when you are running under a hypervisor.  It's not super precise,
   but it's better than what you have.
2. Define a new X86_FEATURE bit for when you are running under
   Hyper-V.
3. Define a new X86_FEATURE bit specifically for Hyper-V isolation VM
   support.  This particular feature might be a little uncommon to
   deserve its own bit.

I'd probably just do #2.