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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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

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