linux-kernel - Re: [PATCH 14/15] KVM: MTRR: do not map huage page for non-consistent range

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Date:	Thu, 04 Jun 2015 16:26:55 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 14/15] KVM: MTRR: do not map huage page for non-consistent
 range



On 06/04/2015 04:23 PM, Xiao Guangrong wrote:
>
>
> On 06/03/2015 03:55 PM, Paolo Bonzini wrote:
>>
>>
>> On 03/06/2015 04:56, Xiao Guangrong wrote:
>>>
>>>
>>> On 06/01/2015 05:36 PM, Paolo Bonzini wrote:
>>>>
>>>>
>>>> On 30/05/2015 12:59, Xiao Guangrong wrote:
>>>>> Currently guest MTRR is completely prohibited if cache snoop is
>>>>> supported on
>>>>> IOMMU (!noncoherent_dma) and host does the emulation based on the
>>>>> knowledge
>>>>> from host side, however, host side is not the good point to know
>>>>> what the purpose of guest is. A good example is that pass-throughed VGA
>>>>> frame buffer is not always UC as host expected
>>>>
>>>> Can you explain how?  The original idea was that such a framebuffer
>>>> would be kvm_is_reserved_pfn and thus be unconditionally UC.
>>>
>>> Yes, frame-buffer is always UC in current code, however, UC for
>>> frame-buffer causes bad performance.
>>
>> Understood now, thanks.
>>
>>> So that guest will configure the range to MTRR, this patchset follows
>>> guest MTRR and cooperates with guest PAT (ept.VMX_EPT_IPAT_BIT = 0) to
>>> emulate guest cache type as guest expects.
>>
>> Unlike e.g. CR0.CD=1, UC memory does not snoop the cache to preserve
>> coherency.  AMD, has special logic to do this, for example:
>>
>> - if guest PAT says "UC" and host MTRR says "WB", the processor will not
>> cache the memory but will snoop the cache as if CR0.CD=1
>>
>> - if guest PAT says "WC" and host (nested page table) PAT says "WB" and
>> host MTRR says "WB", the processor will still do write combining but
>> also snoop the cache as if CR0.CD=1
>>
>> I am worried that the lack of this feature could cause problems if
>> guests map QEMU's VGA framebuffer as uncached.  We have this problem on
>> ARM, so it's not 100% theoretical.
>
> CR0.CD is always 0 in both host and guest, i guess it's why we cleared
> CR0.CD and CR0.NW in vmx_set_cr0().

It reminds me that we should check guest CR0.CD before check guest MTRR
and disable guest PAT if guest CR0.CD = 1.

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