Re: [Xen-devel] Shattering superpages impact on IOMMU in Xen

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 3 Apr 2017, Oleksandr Tyshchenko wrote:
> On Mon, Apr 3, 2017 at 9:06 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> > Hi Andrew,
> >
> >
> > On 03/04/17 18:16, Andrew Cooper wrote:
> >>
> >> On 03/04/17 18:02, Julien Grall wrote:
> >>>
> >>> Hi Andrew,
> >>>
> >>> On 03/04/17 17:42, Andrew Cooper wrote:
> >>>>
> >>>> On 03/04/17 17:24, Oleksandr Tyshchenko wrote:
> >>>>>
> >>>>> Hi, all.
> >>>>>
> >>>>> Playing with non-shared IOMMU in Xen on ARM I faced one interesting
> >>>>> thing. I found out that the superpages were shattered during domain
> >>>>> life cycle.
> >>>>> This is the result of mapping of foreign pages, ballooning memory,
> >>>>> even if domain maps Xen shared pages, etc.
> >>>>> I don't bother with the memory fragmentation at the moment. But,
> >>>>> shattering bothers me from the IOMMU point of view.
> >>>>> As the Xen owns IOMMU it might manipulate IOMMU page tables when
> >>>>> passthoughed/protected device doing DMA in Linux. It is hard to detect
> >>>>> when the DMA transaction isn't in progress
> >>>>> in order to prevent this race. So, if we have inflight transaction
> >>>>> from a device when changing IOMMU mapping we might get into trouble.
> >>>>> Unfortunately, not in all the cases the
> >>>>> faulting transaction can be restarted. The chance to hit the problem
> >>>>> increases during shattering.
> >>>>>
> >>>>> I did next test:
> >>>>> The dom0 on my setup contains ethernet IP that are protected by IOMMU.
> >>>>> What is more, as the IOMMU I am playing with supports superpages (2M,
> >>>>> 1G) the IOMMU driver
> >>>>> takes into account these capabilities when building page tables. As I
> >>>>> gave 256 MB for dom0, the IOMMU mapping was built by 2M memory blocks
> >>>>> only. As I am using NFS for both dom0 and domU the ethernet IP
> >>>>> performs DMA transactions almost all the time.
> >>>>> Sometimes, I see the IOMMU page faults during creating guest domain. I
> >>>>> think, it happens during Xen is shattering 2M mappings 4K mappings (it
> >>>>> unmaps dom0 pages by one 4K page at a time, then maps domU pages there
> >>>>> for copying domU images).
> >>>>> But, I don't see any page faults when the IOMMU page table was built
> >>>>> by 4K pages only.
> >>>>>
> >>>>> I had a talk with Julien on IIRC and we came to conclusion that the
> >>>>> safest way would be to use 4K pages to prevent shattering, so the
> >>>>> IOMMU shouldn't report superpage capability.
> >>>>> On the other hand, if we build IOMMU from 4K pages we will have
> >>>>> performance drop (during building, walking page tables), TLB pressure,
> >>>>> etc.
> >>>>> Another possible solution Julien was suggesting is to always
> >>>>> ballooning with 2M, 1G, and not using 4K. That would help us to
> >>>>> prevent shattering effect.
> >>>>> The discussion was moved to the ML since it seems to be a generic
> >>>>> issue and the right solution should be think of.
> >>>>>
> >>>>> What do you think is the right way to follow? Use 4K pages and don't
> >>>>> bother with shattering or try to optimize? And if the idea to make
> >>>>> balloon mechanism smarter makes sense how to teach balloon to do so?
> >>>>> Thank you.
> >>>>
> >>>>
> >>>> Ballooning and foreign mappings are terrible for trying to retain
> >>>> superpage mappings.  No OS, not even Linux, can sensibly provide victim
> >>>> pages in a useful way to avoid shattering.
> >>>>
> >>>> If you care about performance, don't ever balloon.  Foreign mappings in
> >>>> translated guests should start from the top of RAM, and work upwards.
> >>>
> >>>
> >>> I am not sure to understand this. Can you extend?
> >>
> >>
> >> I am not sure what is unclear.  Handing random frames of RAM back to the
> >> hypervisor is what exacerbates host superpage fragmentation, and all
> >> balloon drivers currently do it.
> >>
> >> If you want to avoid host superpage fragmentation, don't use a
> >> scattergun approach of handing frames back to Xen.  However, because
> >> even Linux doesn't provide enough hooks into the physical memory
> >> management logic, the only solution is to not balloon at all, and to use
> >> already-unoccupied frames for foreign mappings.
> >
> >
> > Do you have any pointer in the Linux code?
> >
> >
> >>
> >>>
> >>>>
> >>>>
> >>>> As for the IOMMU specifically, things are rather easier.  It is the
> >>>> guests responsibility to ensure that frames offered up for ballooning or
> >>>> foreign mappings are unused.  Therefore, if anything cares about the
> >>>> specific 4K region becoming non-present in the IOMMU mappings, it is the
> >>>> guest kernels fault for offering up a frame already in use.
> >>>>
> >>>> For the shattering however, It is Xen's responsibility to ensure that
> >>>> all other mappings stay valid at all points.  The correct way to do this
> >>>> is to construct a new L1 table, mirroring the L2 superpage but lacking
> >>>> the specific 4K mapping in question, then atomically replace the L2
> >>>> superpage entry with the new L1 table, then issue an IOMMU TLB
> >>>> invalidation to remove any cached mappings.
> >>>>
> >>>> By following that procedure, all DMA within the 2M region, but not
> >>>> hitting the 4K frame, won't observe any interim lack of mappings.  It
> >>>> appears from your description that Xen isn't following the procedure.
> >>>
> >>>
> >>> Xen is following what's the ARM ARM is mandating. For shattering page
> >>> table, we have to follow the break-before-sequence i.e:
> >>>     - Invalidate the L2 entry
> >>>     - Flush the TLBs
> >>>     - Add the new L1 table
> >>> See D4-1816 in ARM DDI 0487A.k_iss10775 for details. So we end up in a
> >>> small window where there are no valid mapping. It is easy to trap data
> >>> abort from processor and restarting it but not for device memory
> >>> transactions.
> >>>
> >>> Xen by default is sharing stage-2 page tables with between the IOMMU
> >>> and the MMU. However, from the discussion I had with Oleksandr, they
> >>> are not sharing page tables and still see the problem. I am not sure
> >>> how they are updating the page table here. Oleksandr, can you provide
> >>> more details?
> >>
> >>
> >> Are you saying that ARM has no way of making atomic updates to the IOMMU
> >> mappings?  (How do I get access to that document?  Google gets me to
> >>
> >> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.subset.architecture.reference/index.html,
> >> but
> >> http://infocenter.arm.com/help/topic/com.arm.doc.ddi0487a.k/index.html
> >> which looks like the document you specified results in 404.)
> >
> >
> > Below a link, I am not sure why google does not refer it:
> >
> > http://infocenter.arm.com/help/topic/com.arm.doc.ddi0487a.k_10775/index.html
> >
> >>
> >> If so, that is an architecture bug IMO.  By design, the IOMMU is out of
> >> control of guest software, and the hypervisor should be able to make
> >> atomic modifications without guest cooperation.
> >
> >
> > I think you misread what I meant, IOMMU supports atomic operations. However,
> > if you share the page table we have to apply Break-Before-Make when
> > shattering superpage. This is mandatory if you want to get Xen running on
> > all the micro-architectures.
> >
> > Some IOMMU may cope with the BBM, some not. I haven't seen any issue so far
> > (it does not mean there are none).
> >
> > The IOMMU used by Oleksandr (e.g VMSA-IPMMU) is an IP from Renesas which I
> > never used myself. In his case he needs different page tables because the
> > layouts are not the same.
> >
> > Oleksandr, looking at the code your provided, the superpage are split the
> > way Andrew said, i.e:
> >         1) allocating level 3 table minus the 4K mapping
> >         2) replace level 2 entry with the new table
> >
> > Am I right?
> 
> It seems, yes. Walking the page table down when trying to unmap we
> bump into leaf entry (2M mapping),
> so 2M-4K mapping are inserted at the next level and after that the
> page table entry are replaced.

Let me premise that Andrew well pointed out what should be the right
approach on dealing with this issue. However, if we have to use
break-before-make for IOMMU pagetables, then it means we cannot do
atomic updates to IOMMU mappings, like Andrew wrote. Therefore, we
have to make a choice: we either disable superpage IOMMU mappings or
ballooning. I would disable IOMMU superpage mappings, on the ground that
supporting superpage mappings without supporting atomic shattering or
restartable transactions is not really supporting superpage mappings.

However, you are not doing break-before-make here. I would investigate
if break-before-make is required by VMSA-IPMMU. If it is not required,
why are you seeing DMA faults?

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