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Re: [Qemu-devel] Hotplug ram and vhost-user
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] Hotplug ram and vhost-user |
Date: |
Thu, 7 Dec 2017 16:25:47 +0000 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
* 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?
> > 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 <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?
> 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 / address@hidden / Manchester, UK
> >
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK