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

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Message-ID: <0d956a05-7d24-57a0-f4a9-dccc849b52fc@gmail.com>
Date:   Thu, 29 Jul 2021 21:01:49 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        konrad.wilk@...cle.com, boris.ostrovsky@...cle.com,
        jgross@...e.com, sstabellini@...nel.org, joro@...tes.org,
        will@...nel.org, davem@...emloft.net, kuba@...nel.org,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, arnd@...db.de,
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        michael.h.kelley@...rosoft.com
Cc:     iommu@...ts.linux-foundation.org, linux-arch@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
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Subject: 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:
>> @@ -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.