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

[Maxime Coquelin]

On 12/07/2017 05:25 PM, Dr. David Alan Gilbert wrote:
> * Maxime Coquelin (address@hidden) 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?

We have a known issue in DPDK, the PMD threads might be accessing the
guest memory while the vhost-user protocol thread is unmapping it.

We have a similar problem with dirty logging area, and Victor is working
on a patch that will fix both issues.

Once ready, I'll have a try and let you know.

> > > 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?

Yes, you may be right. I think it is worth to debug it to understand
what is going on.

> > 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 <address@hidden>
> > 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 <address@hidden>
> > 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?

vhost-user-test, but it should no more be failing now that it no more
uses TCG.

I think we could consider reverting the revert. i.e. send get_features
in set_mem_table toi be sure it has been handled.

> > 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!

Sure.

Maxime
> Dave
>
> > Maxime
> > > Dave
> > >
> > > --
> > > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK