linux-kernel - Re: [PATCH 07/15] KVM: MTRR: improve kvm_mtrr_get_guest_memory

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Date:	Wed, 03 Jun 2015 09:57:16 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
CC:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/15] KVM: MTRR: improve kvm_mtrr_get_guest_memory_type



On 03/06/2015 04:12, Xiao Guangrong wrote:
>>
>> Perhaps just use type = MIN(type, curr_type), which also happens to get
>> WT vs. WB right?  You can also add a
>>
>>     BUILD_BUG_ON(MTRR_TYPE_WRTHROUGH > MTRR_TYPE_WRBACK);
>>
>> to ensure that the WT vs. WB precedence is correct.
> 
> Only WT and WB are allowed to be overlapped here otherwise is
> "undefined behavior". For example if the types are MTRR_TYPE_WRPROT
> and MTRR_TYPE_WRTHROUGH, min() will get MTRR_TYPE_WRTHROUGH rather than
> "undefined behavior".

Choosing MTRR_TYPE_WRTHROUGH is one example of achieve undefined
behavior.  Picking one type arbitrarily is just a different kind of
undefined behavior.

Paolo
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