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Subject: Re: [RESEND PATCH v3] KVM: x86: Add Intel CPUID.1F cpuid emulation
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To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org
References: <20190526133052.4069-1-like.xu@linux.intel.com>
 <20190603165616.GA11101@flask> <20190603191818.GF13384@linux.intel.com>
From:   Like Xu <like.xu@linux.intel.com>
Organization: Intel OTC
Message-ID: <5e49ff48-9071-1419-ee36-bbc857164c28@linux.intel.com>
Date:   Thu, 6 Jun 2019 09:30:46 +0800
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On 2019/6/4 3:18, Sean Christopherson wrote:
> On Mon, Jun 03, 2019 at 06:56:17PM +0200, Radim Krčmář wrote:
>>> +			break;
>>> +		}
>>>   		entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
>>
>> Similarly in the existing code.  If we don't have f_intel_pt, then we
>> should make sure that leaf 0x14 is not being filled, but we don't really
>> have to limit the maximal index.
>>
>> Adding a single clamping like
>>
>> 		/* Limited to the highest leaf implemented in KVM. */
>> 		entry->eax = min(entry->eax, 0x1f);
>>
>> seems sufficient.
>>
>> (Passing the hardware value is ok in theory, but it is a cheap way to
>>   avoid future leaves that cannot be simply zeroed for some weird reason.)
> 
> I don't have a strong opinion regarding the code itself, but whatever ends
> up getting committed should have a big beefy changelog explaining why the
> clamping exists, or at least extolling its virtues.  I had a hell of a
> time understanding the intent of this one line of code because as your
> response shows, there is no one right answer.
> 

Hi Radim & Sean,

Thanks for your review and finally we find a better way.

Please help review the new version, I am not sure that the changelog 
could help new submitter understand why the clamping exists.