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Subject: Re: [PATCH 03/13] x86/HV: Add new hvcall guest address host
 visibility support
To:     Dave Hansen <dave.hansen@intel.com>, kys@microsoft.com,
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References: <20210728145232.285861-1-ltykernel@gmail.com>
 <20210728145232.285861-4-ltykernel@gmail.com>
 <a2444c36-0103-8e1c-7005-d97f77f90e85@intel.com>
 <0d956a05-7d24-57a0-f4a9-dccc849b52fc@gmail.com>
 <ec1d4cfd-bbbc-e27a-7589-e85d9f0438f4@intel.com>
From:   Tianyu Lan <ltykernel@gmail.com>
Message-ID: <8df2845d-ee90-56d0-1228-adebb103ec37@gmail.com>
Date:   Thu, 29 Jul 2021 23:02:16 +0800
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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
>     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.
> 

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();
}