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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>
From:   Tianyu Lan <ltykernel@gmail.com>
Message-ID: <0d956a05-7d24-57a0-f4a9-dccc849b52fc@gmail.com>
Date:   Thu, 29 Jul 2021 21:01:49 +0800
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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.