Re: VM crashed while hot-plugging memory

[Igor Mammedov]

On Fri, 10 Feb 2023 09:30:18 +0000
Yangming via <qemu-devel@nongnu.org> wrote:

> Hello all:
> 
> I found VM crashed while hot-plugging memory.
> 
> Base infomation:
> qemu version: qemu-master
> requirements: hugepages, virtio-gpu
> 
> It happens by the following steps:
> 1. Booting a VM with hugepages and a virtio-gpu device.
> 2. Connecting VNC of the VM.
> 3. After the VM booted, hot-plugging 512G memory.
> 4. Then you can find that the image in vnc is blocked and the worse thing is 
> that the VM crashed.
> 
> Actually the vcpu is blocked because of dead lock.
> 
> Analysis:
> As when hot-pluging the BQL is held, at the meanwhile, virtio-gpu is trying 
> to hold the BQL for writing date. Then a vcpu is blocked waiting for 
> hugepages hot-plugging, specifically, waiting for touching pages. If the 
> blocked vcpu stops for several seconds, the soft lockup will happen, if it 
> stops for a long time, e.g. 30s, the VM will crash.
> 
> I am wandering if there are some ideas to avoid VM soft lockup and even VM 
> crash ?

Maybe David can suggest something
(CCed)

> 
> Thank you!
> kind regards!