Messages in this thread |  | | Date | Wed, 03 Jun 2015 10:56:56 +0800 | From | Xiao Guangrong <> | Subject | Re: [PATCH 14/15] KVM: MTRR: do not map huage page for non-consistent range |
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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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