[Qemu-devel] [Bug 1762707] Re: VFIO device gets DMA failures when virtio-balloon leak from highmem to lowmem

[Jose Carlos Venegas Munoz]

Alex, I see this issue is closed but I have a question, do you know if
the problem only comes the balloon is resized to return memory to the
host. I ask because we have a situation where we will start a VM with
balloon enabled, and later it maybe possible a devices is assigned via
hot-plug. So I would like to avoid this issue by doing the following:

if a vfio devices is assigned;
   resize the balloon size the the maximal guest memory
end 

Then because we know we added a vfio devices never resize the balloon to
return memory again.

More information about what we want to do: https://github.com/kata-
containers/runtime/pull/793

Regards,
Carlos

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https://bugs.launchpad.net/bugs/1762707

Title:
  VFIO device gets DMA failures when virtio-balloon leak from highmem to
  lowmem

Status in QEMU:
  Invalid

Bug description:
  Is there any known conflict between VFIO passthrough device and
  virtio-balloon?

  The VM has:
  1. 4GB system memory
  2. one VFIO passthrough device which supports high address memory DMA and 
uses GFP_HIGHUSER pages.
  3. Memory balloon device with 4GB target.

  When setting the memory balloon target to 1GB and 4GB in loop during
  runtime (I used the command "virsh qemu-monitor-command debian --hmp
  --cmd balloon 1024"), the VFIO device DMA randomly gets failure.

  More clues:
  1. configure 2GB system memory (no highmem) VM, no issue with similar 
operations
  2. setting the memory balloon to higher like 8GB, no issue with similar 
operations

  I'm also trying to narrow down this issue. It's appreciated for that
  you guys may share some thoughts.

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