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

On Thu, Nov 26, 2015 at 01:55:57PM +0000, Andrew Cooper wrote:
> 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.

How did you observe this? I am mighty curious what kind of performance tools
you used to find this  as I would love to figure out if some of the issues
we have seen are related to this?

> ~Andrew
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