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

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Date:	Wed, 03 Jun 2015 10:56:56 +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 06/01/2015 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. It's quoted from Documentation/x86/mtrr.txt:

| This is most useful when you have a video (VGA) card on a PCI or AGP bus.
| Enabling write-combining allows bus write transfers to be combined into a
| larger transfer before bursting over the PCI/AGP bus. This can increase
| performance of image write operations 2.5 times or more.

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.
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