Re: [Qemu-devel] Hotplug ram and vhost-user

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi David,

On 12/05/2017 06:41 PM, Dr. David Alan Gilbert wrote:
> Hi,
>    Since I'm reworking the memory map update code I've been
> trying to test it with hot adding RAM; but even on upstream
> I'm finding that hot adding RAM causes the guest to stop passing
> packets with vhost-user-bridge;  have either of you seen the same
> thing?

No, I have never tried this.

> I'm doing:
> ./tests/vhost-user-bridge -u /tmp/vubrsrc.sock
> $QEMU -enable-kvm -m 1G,maxmem=2G,slots=4 -smp 2 -object memory-backend-file,id=mem,size=1G,mem-path=/dev/shm,share=on -numa node,memdev=mem -mem-prealloc -trace events=vhost-trace-file -chardev socket,id=char0,path=/tmp/vubrsrc.sock -netdev type=vhost-user,id=mynet1,chardev=char0,vhostforce -device virtio-net-pci,netdev=mynet1 $IMAGE -net none
> 
> (with a f27 guest) and then doing:
> (qemu) object_add memory-backend-file,id=mem1,size=256M,mem-path=/dev/shm
> (qemu) device_add pc-dimm,id=dimm1,memdev=mem1
> 
> but then not getting any responses inside the guest.
> 
> I can see the code sending another set-mem-table with the
> extra chunk of RAM and fd, and I think I can see the bridge
> mapping it.

I think there are at least two problems.
The first one is that vhost-user-bridge does not support vhost-user
protocol's reply-ack feature. So when QEMU sends the requests, it cannot 
know whether/when it has been handled by the backend.

It had been fixed by sending a GET_FEATURE requests to be sure the 
SET_MEM_TABLE was handled, as messages are processed in order. The 
problem is that it caused some test failures when using TCG, so it got
reverted.

The initial fix:

commit 28ed5ef16384f12500abd3647973ee21b03cbe23
Author: Prerna Saxena <prerna.saxena@nutanix.com>
Date:   Fri Aug 5 03:53:51 2016 -0700

     vhost-user: Attempt to fix a race with set_mem_table.

The revert:

commit 94c9cb31c04737f86be29afefbff401cd23bc24d
Author: Michael S. Tsirkin <mst@redhat.com>
Date:   Mon Aug 15 16:35:24 2016 +0300

     Revert "vhost-user: Attempt to fix a race with set_mem_table."


Another problem is that memory mmapped with previous call does not seems
to be unmapped, but that should not cause other problems than leaking
virtual memory.

Maxime
> Dave
> 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>