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

[Julien Grall <julien.grall@xxxxxxx>]

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?

> 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?
Cheers,

--
Julien Grall

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