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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Maxime Coquelin (maxime.coquelin@redhat.com) wrote:
> 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.

Would you know if it works on dpdk?

> > 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.

Wouldn't you have to be unlucky to cause that a problem - i.e. the
descriptors would have to get allocated in the new RAM?

> 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."
> 

Do we know which tests fail?

> 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.

Oh, leaks are the least of our problem there!

Dave

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