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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Thursday, December 03, 2015 7:24 PM
> 
> 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.
> 

Yes, it makes more sense.

Thanks
Kevin

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