[vfio-users] qemu stuck when hot-add memory to a virtual machine with a device passthrough

[Wuzongyong (Euler Dept)]

> > Hi,
> >
> > The qemu process will  stuck when hot-add large size  memory to the
> > virtual machine with a device passtrhough.
> > We found it is too slow to pin and map pages in vfio_dma_do_map.
> > Is there any method to improve this process?
> 
> At what size do you start to see problems?  The time to map a section of
> memory should be directly proportional to the size.  As the size is
> increased, it will take longer, but I don't know why you'd reach a point
> of not making forward progress.  Is it actually stuck or is it just taking
> longer than you want?  Using hugepages can certainly help, we still need
> to pin each PAGE_SIZE page within the hugepage, but we'll have larger
> contiguous regions and therefore call iommu_map() less frequently.  Please
> share more data.  Thanks,
> 
> Alex
It just take longer time, instead of actually stuck. 
We found that the problem exist when we hot-added 16G memory. And it will consume
tens of minutes when we hot-added 1T memory.

Thanks,
Wu Zongyong