[Xen-devel] Memory fragmentation and PCI passthrough

[Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

Hello,

I've hit known problem with dynamic memory management - memory
fragmentation... This dynamic memory management basically does xl
mem-set to balance memory.

After some time of running system, xen memory is so fragmented that it
is impossible to start new VM with PCI device. Sometimes it crashes
during boot (no 64MB contiguous memory for SWIOTLB), or later - eg.
iwlagn cannot allocate memory for loading firmware (few allocs, each
bellow 100k).

DomU kernel cmdline: console=hvc0 iommu=soft earlyprintk=xen

There is two cases (I think):
1. With IOMMU
2. Without IOMMU
I've tried only the second one.

Is there any known solution for this problem?
Some ideas:
1. With IOMMU pass iommu=pv to Xen. AFAIU domU will not need iommu=soft
parameter then, right? Will it work then with fragmented memory?

2. Force somehow on xen/libxl to allocate memory (for domU) in chunks
of, say 4MB, to not fragment it so badly. Is it doable?

In tmem documentation is also described some workaround for this:
reserve some memory region for allocations with 0<order<=9. But SWIOTLB
tries to allocate 64MB, which much bigger than 2MB... Is it really
needed to allocate such big region of contiguous memory in one piece?

-- 
Pozdrawiam / Best Regards,
Marek Marczykowski
Invisible Things Lab

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