[Qemu-devel] Hotplug ram and vhost-user

[Qemu-devel] Hotplug ram and vhost-user

[Dr. David Alan Gilbert]

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?

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.

Dave

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

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

[Maxime Coquelin]

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
> 

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

[Michael S. Tsirkin]

On Thu, Dec 07, 2017 at 04:52:18PM +0100, Maxime Coquelin 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.
> 
> > 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."

It's a question of stress-testing it and finding out why did
it cause tests fail esp when run within a container.

> 
> 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
> > 

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

[Dr. David Alan Gilbert]

* 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

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

[Maxime Coquelin]

On 12/07/2017 04:56 PM, Michael S. Tsirkin wrote:
> On Thu, Dec 07, 2017 at 04:52:18PM +0100, Maxime Coquelin 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.
>>
>>> 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."
> 
> It's a question of stress-testing it and finding out why did
> it cause tests fail esp when run within a container.

Actually I did work on fixing it last year, and proposed below series:
http://lists.gnu.org/archive/html/qemu-devel/2016-09/msg01704.html

It felt through the cracks though. Maybe we could just revert your
revert (patch 1 of my series) now that TCG is no more used by vhost-
user-test?

Maxime
>>
>> 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
>>>

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

[Maxime Coquelin]

On 12/07/2017 05:25 PM, Dr. David Alan Gilbert wrote:
> * 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?

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 <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?

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

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

[Michael S. Tsirkin]

On Thu, Dec 07, 2017 at 05:35:13PM +0100, Maxime Coquelin wrote:
> 
> 
> On 12/07/2017 04:56 PM, Michael S. Tsirkin wrote:
> > On Thu, Dec 07, 2017 at 04:52:18PM +0100, Maxime Coquelin 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.
> > > 
> > > > 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."
> > 
> > It's a question of stress-testing it and finding out why did
> > it cause tests fail esp when run within a container.
> 
> Actually I did work on fixing it last year, and proposed below series:
> http://lists.gnu.org/archive/html/qemu-devel/2016-09/msg01704.html
> 
> It felt through the cracks though. Maybe we could just revert your
> revert (patch 1 of my series) now that TCG is no more used by vhost-
> user-test?
> 
> Maxime

TCG just changed timing making races more obvious.
And it's still used on many ci systems.

> > > 
> > > 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
> > > > 

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

[Michael S. Tsirkin]

On Thu, Dec 07, 2017 at 05:35:13PM +0100, Maxime Coquelin wrote:
> 
> 
> On 12/07/2017 04:56 PM, Michael S. Tsirkin wrote:
> > On Thu, Dec 07, 2017 at 04:52:18PM +0100, Maxime Coquelin 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.
> > > 
> > > > 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."
> > 
> > It's a question of stress-testing it and finding out why did
> > it cause tests fail esp when run within a container.
> 
> Actually I did work on fixing it last year, and proposed below series:
> http://lists.gnu.org/archive/html/qemu-devel/2016-09/msg01704.html
> 
> It felt through the cracks though. Maybe we could just revert your
> revert (patch 1 of my series) now that TCG is no more used by vhost-
> user-test?
> 
> Maxime

Canyou pls explain this:

Analysis of the race shows that it would happen only when QEMU relies
on TCG.

what is the reason it only happens with tcg?

> > > 
> > > 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
> > > > 

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

[Dr. David Alan Gilbert]

* Maxime Coquelin (maxime.coquelin@redhat.com) wrote:
> 
> 
> On 12/07/2017 05:25 PM, Dr. David Alan Gilbert wrote:
> > * 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?
> 
> 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 <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?
> 
> 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.

How does it fail? Does it fail every time or only some times?
(The postcopy test in migration-test.c also fails under TCG under
very heavy load and I've not figured out why yet).

Dave

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

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

[Maxime Coquelin]

On 12/07/2017 07:23 PM, Dr. David Alan Gilbert wrote:
> * Maxime Coquelin (maxime.coquelin@redhat.com) wrote:
>>
>>
>> On 12/07/2017 05:25 PM, Dr. David Alan Gilbert wrote:
>>> * 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?
>>
>> 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 <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?
>>
>> 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.
> 
> How does it fail? Does it fail every time or only some times?
> (The postcopy test in migration-test.c also fails under TCG under
> very heavy load and I've not figured out why yet).

I'm trying to remember the analysis I did one year ago... I don't have
yet the full picture, but found some notes I took at that time:

"
I have managed to reproduce the hang by adding some debug prints into
vhost_user_get_features().

Doing this the issue is reproducible quite easily.
Another way to reproduce it in one shot is to strace (with following
forks) /vhost-user-test execution.

So, by adding debug prints at vhost_user_get_features() entry and exit,
we can see we never return from this function when hang happens.
Strace of Qemu instance shows that its thread keeps retrying to receive
GET_FEATURE reply:

write(1, "vhost_user_get_features IN: \n", 29) = 29
sendmsg(11, {msg_name=NULL, msg_namelen=0,
         msg_iov=[{iov_base="\1\0\0\0\1\0\0\0\0\0\0\0", iov_len=12}],
         msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 12
recvmsg(11, {msg_namelen=0}, MSG_CMSG_CLOEXEC) = -1 EAGAIN
nanosleep({0, 100000}, 0x7fff29f8dd70)  = 0
...
recvmsg(11, {msg_namelen=0}, MSG_CMSG_CLOEXEC) = -1 EAGAIN
nanosleep({0, 100000}, 0x7fff29f8dd70)  = 0

The reason is that vhost-user-test never replies to Qemu,
because its thread handling the GET_FEATURES command is waiting for
the s->data_mutex lock.
This lock is held by the other vhost-user-test thread, executing
read_guest_mem().

The lock is never released because the thread is blocked in read
syscall, when read_guest_mem() is doing the readl().

This is because on Qemu side, the thread polling the qtest socket is
waiting for the qemu_global_mutex (in os_host_main_loop_wait()), but
the mutex is held by the thread trying to get the GET_FEATURE reply
(the TCG one).
"

It does not explain why it would only fail with TCG, I would need to
spend some time investigating the issue to find why I claimed this.

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

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

[Dr. David Alan Gilbert]

* Maxime Coquelin (maxime.coquelin@redhat.com) wrote:
> 
> 
> On 12/07/2017 07:23 PM, Dr. David Alan Gilbert wrote:
> > * Maxime Coquelin (maxime.coquelin@redhat.com) wrote:
> > > 
> > > 
> > > On 12/07/2017 05:25 PM, Dr. David Alan Gilbert wrote:
> > > > * 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?
> > > 
> > > 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 <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?
> > > 
> > > 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.
> > 
> > How does it fail? Does it fail every time or only some times?
> > (The postcopy test in migration-test.c also fails under TCG under
> > very heavy load and I've not figured out why yet).
> 
> I'm trying to remember the analysis I did one year ago... I don't have
> yet the full picture, but found some notes I took at that time:
> 
> "
> I have managed to reproduce the hang by adding some debug prints into
> vhost_user_get_features().
> 
> Doing this the issue is reproducible quite easily.
> Another way to reproduce it in one shot is to strace (with following
> forks) /vhost-user-test execution.
> 
> So, by adding debug prints at vhost_user_get_features() entry and exit,
> we can see we never return from this function when hang happens.
> Strace of Qemu instance shows that its thread keeps retrying to receive
> GET_FEATURE reply:
> 
> write(1, "vhost_user_get_features IN: \n", 29) = 29
> sendmsg(11, {msg_name=NULL, msg_namelen=0,
>         msg_iov=[{iov_base="\1\0\0\0\1\0\0\0\0\0\0\0", iov_len=12}],
>         msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 12
> recvmsg(11, {msg_namelen=0}, MSG_CMSG_CLOEXEC) = -1 EAGAIN
> nanosleep({0, 100000}, 0x7fff29f8dd70)  = 0
> ...
> recvmsg(11, {msg_namelen=0}, MSG_CMSG_CLOEXEC) = -1 EAGAIN
> nanosleep({0, 100000}, 0x7fff29f8dd70)  = 0
> 
> The reason is that vhost-user-test never replies to Qemu,
> because its thread handling the GET_FEATURES command is waiting for
> the s->data_mutex lock.
> This lock is held by the other vhost-user-test thread, executing
> read_guest_mem().
> 
> The lock is never released because the thread is blocked in read
> syscall, when read_guest_mem() is doing the readl().
> 
> This is because on Qemu side, the thread polling the qtest socket is
> waiting for the qemu_global_mutex (in os_host_main_loop_wait()), but
> the mutex is held by the thread trying to get the GET_FEATURE reply
> (the TCG one).
> "
> 
> It does not explain why it would only fail with TCG, I would need to
> spend some time investigating the issue to find why I claimed this.

That sounds like a very repeatable bug; much more so than my postcopy
case;  is your case that it's just anything that slows it down - be it
some debug, an strace or a slow tcg?

Dave

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