Re: [Xen-devel] Bug: Limitation of <=2GB RAM in domU persists with 4.3.0

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, Jul 24, 2013 at 11:15 PM, Gordan Bobic <gordan@xxxxxxxxxx> wrote:
> Attached are the logs (loglvl=all) and configs for 2GB (working) and 8GB
> (screen corruption + domU crash + sometimes dom0 crashing with it).
>
> I can see in the xl-dmesg log in 8GB case that there is memory remapping
> going on to allow for the lowmem MMIO hole, but it doesn't seem to help.

Gordan,

There's a possibility that it's actually got nothing to do with
relocation, but with bugs in your hardware.  Can you try:
* Set the guest memory to 3600
* Boot the guest, and check to make sure that xl dmesg shows does
*not* relocate memory?
* Report whether it crashes?

If it's a bug in the hardware, I would expect to see that memory was
not relocated, but that the system will lock up anyway.

Can you also do lspci -vvv in dom0 before assigning the device and
attach the output?

The hardware bug we've seen is this: In order for the IOMMU to work
properly, *all* DMA transactions must be passed up to the root bridge
so the IOMMU can translate the addresses from guest address to host
address.  Unfortunately, an awful lot of bridges will not do this
properly, which means that the address is not translated properly,
which means that if a *guest* memory address overlaps the a *host*
MMIO range, badness ensues.  There's nothing we can do about this in
Xen other than make the guest MMIO hole the same size as the host MMIO
hole.

Thanks,
 -George

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