Re: [Xen-devel] [PATCH] iommu/quirk: disable shared EPT for Sandybridge and earlier processors.

[Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>]

On 26/11/15 13:46, Jan Beulich wrote:
>>>> On 25.11.15 at 11:28, <andrew.cooper3@xxxxxxxxxx> wrote:
>> The problem is that SandyBridge IOMMUs advertise 2M support and do
>> function with it, but cannot cache 2MB translations in the IOTLBs.
>>
>> As a result, attempting to use 2M translations causes substantially
>> worse performance than 4K translations.
> 
> Btw - how does this get explained? At a first glance, even if 2Mb
> translations don't get entered into the TLB, it should still be one
> less page table level to walk for the IOMMU, and should hence
> nevertheless be a benefit. Yet you even say _substantially_
> worse performance results.

There is a IOTLB for the 4K translation so if you only use 4K
translations then you get to take advantage of the IOTLB.

If you use the 2Mb translation then a page table walk has to be
performed every time there's a DMA access to that region of the BFN
address space.

Malcolm

> 
> Jan
> 


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