linux-kernel - Re: [PATCH 1/9] KVM: MMU: fix decoding cache type from MTRR

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Message-ID: <554AC8EE.50607@linux.intel.com>
Date:	Thu, 07 May 2015 10:07:42 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	David Matlack <dmatlack@...gle.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	Gleb Natapov <gleb@...nel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	kvm list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/9] KVM: MMU: fix decoding cache type from MTRR



On 05/07/2015 05:42 AM, David Matlack wrote:
> On Thu, Apr 30, 2015 at 3:24 AM,  <guangrong.xiao@...ux.intel.com> wrote:
>> From: Xiao Guangrong <guangrong.xiao@...ux.intel.com>
>>
>> There are some bugs in current get_mtrr_type();
>> 1: bit 2 of mtrr_state->enabled is corresponding bit 11 of IA32_MTRR_DEF_TYPE
>
> bit 1, not bit 2. (code is correct though)

Oh, i counted the bit from 1, my fault. :(

>
>>     MSR which completely control MTRR's enablement that means other bits are
>>     ignored if it is cleared
>>
>> 2: the fixed MTRR ranges are controlled by bit 1 of mtrr_state->enabled (bit 10
>
> bit 0, not bit 1. (code is correct though)

Ditto. Will update the changelog in v2. Thank you for pointing it out.

>
>>     of IA32_MTRR_DEF_TYPE)
>>
>> 3: if MTRR is disabled, UC is applied to all of physical memory rather than
>>     mtrr_state->def_type
>
> kvm_get_guest_memory_type defaults to MTRR_TYPE_WRBACK, not
> mtrr_state->def_type, when get_mtrr_type returns 0xFF.
>

Yeah, that confused me. Based on the comment of vmx_get_mt_mask():
	 *   a. VT-d without snooping control feature: can't guarantee the
	 *	result, try to trust guest.
we need to completely follow guest's MTRR under this case.
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