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

[Alex Williamson]

On Wed, 18 Apr 2018 10:19:32 +0000
"Wuzongyong (Euler Dept)" <cordius.wu at huawei.com> wrote:

> 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