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

On 7/29/2021 10:09 PM, Dave Hansen wrote:
On 7/29/21 6:01 AM, Tianyu Lan wrote:
On 7/29/2021 1:06 AM, Dave Hansen wrote:
On 7/28/21 7:52 AM, Tianyu Lan wrote:
@@ -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
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.

There is x86_hyper_type to identify hypervisor type and we may check this variable after checking X86_FEATURE_HYPERVISOR.

static inline bool hv_is_isolation_supported(void)
        if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
                return 0;

        if (x86_hyper_type != X86_HYPER_MS_HYPERV)
                return 0;

        // out of line function call:
        return __hv_is_isolation_supported();



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