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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/12/15 01:19, Tian, Kevin wrote:
>> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
>> Sent: Thursday, November 26, 2015 9:56 PM
>>
>> On 26/11/15 13:48, Malcolm Crossley wrote:
>>> 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.
>> Also remember that a high level dma access (from the point of view of a
>> driver) will be fragmented at the PCIe max packet size, which is
>> typically 256 bytes.
>>
>> So by not caching the 2Mb translation, a dma access of 4k may undergo 16
>> pagetable walks, one for each PCIe packet.
>>
>> We observed that using 2Mb mappings results in a 40% overhead, compared
>> to using 4k mappings, from the point of view of a sample network workload.
>>
>> ~Andrew
> One confusion here. The original patch just disables shared_ept, w/o
> changing IOMMU to not use 2MB mapping. Is there something missing
> or other tricks behind?

Disabling shared_ept works because the Xen IOMMU interface doesn't
support superpages.

However, I subsequently changed my mind in this thread about the
approach taken, and quirking the IOMMUs not to report superpage
capabilities is the correct solution to the problem.

~Andrew

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