Re: [Qemu-devel] vhost-net issue: does not survive reboot on ppc64

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Dec 23, 2013 at 02:01:13AM +1100, Alexey Kardashevskiy wrote:
> On 12/23/2013 01:46 AM, Alexey Kardashevskiy wrote:
> > On 12/22/2013 09:56 PM, Michael S. Tsirkin wrote:
> >> On Sun, Dec 22, 2013 at 02:01:23AM +1100, Alexey Kardashevskiy wrote:
> >>> Hi!
> >>>
> >>> I am having a problem with virtio-net + vhost on POWER7 machine - it does
> >>> not survive reboot of the guest.
> >>>
> >>> Steps to reproduce:
> >>> 1. boot the guest
> >>> 2. configure eth0 and do ping - everything works
> >>> 3. reboot the guest (i.e. type "reboot")
> >>> 4. when it is booted, eth0 can be configured but will not work at all.
> >>>
> >>> The test is:
> >>> ifconfig eth0 172.20.1.2 up
> >>> ping 172.20.1.23
> >>>
> >>> If to run tcpdump on the host's "tap-id3" interface, it shows no trafic
> >>> coming from the guest. If to compare how it works before and after reboot,
> >>> I can see the guest doing an ARP request for 172.20.1.23 and receives the
> >>> response and it does the same after reboot but the answer does not come.
> >>
> >> So you see the arp packet in guest but not in host?
> > 
> > Yes.
> > 
> > 
> >> One thing to try is to boot debug kernel - where pr_debug is
> >> enabled - then you might see some errors in the kernel log.
> > 
> > Tried and added lot more debug printk myself, not clear at all what is
> > happening there.
> > 
> > One more hint - if I boot the guest and the guest does not bring eth0 up
> > AND wait more than 200 seconds (and less than 210 seconds), then eth0 will
> > not work at all. I.e. this script produces not-working-eth0:
> > 
> > 
> > ifconfig eth0 172.20.1.2 down
> > sleep 210
> > ifconfig eth0 172.20.1.2 up
> > ping 172.20.1.23
> > 
> > s/210/200/ - and it starts working. No reboot is required to reproduce.
> > 
> > No "vhost" == always works. The only difference I can see here is vhost's
> > thread which may get suspended if not used for a while after the start and
> > does not wake up but this is almost a blind guess.
> 
> 
> Yet another clue - this host kernel patch seems to help with the guest
> reboot but does not help with the initial 210 seconds delay:
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 69068e0..5e67650 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -162,10 +162,10 @@ void vhost_work_queue(struct vhost_dev *dev, struct
> vhost_work *work)
>                 list_add_tail(&work->node, &dev->work_list);
>                 work->queue_seq++;
>                 spin_unlock_irqrestore(&dev->work_lock, flags);
> -               wake_up_process(dev->worker);
>         } else {
>                 spin_unlock_irqrestore(&dev->work_lock, flags);
>         }
> +       wake_up_process(dev->worker);
>  }
>  EXPORT_SYMBOL_GPL(vhost_work_queue);
> 
> 

Interesting. Some kind of race? A missing memory barrier somewhere?

Since it's all around startup,
you can try kicking the host eventfd in
vhost_net_start.

> 
> 
> >>> If to remove vhost=on, it is all good. If to try Fedora19
> >>> (v3.10-something), it all good again - works before and after reboot.
> >>>
> >>>
> >>> And there 2 questions:
> >>>
> >>> 1. does anybody have any clue what might go wrong after reboot?
> >>>
> >>> 2. Is there any good material to read about what exactly and how vhost
> >>> accelerates?
> >>>
> >>> My understanding is that packets from the guest to the real network are
> >>> going as:
> >>> 1. guest's virtio-pci-net does ioport(VIRTIO_PCI_QUEUE_NOTIFY)
> >>> 2. QEMU's net/virtio-net.c calls qemu_net_queue_deliver()
> >>> 3. QEMU's net/tap.c calls tap_write_packet() and this is how the host knows
> >>> that there is a new packet.
> > 
> > 
> > What about the documentation? :) or the idea?
> > 
> > 
> >>>
> >>>
> >>> Thanks!
> >>>
> >>>
> >>> This how I run QEMU:
> >>> ./qemu-system-ppc64 \
> >>> -enable-kvm \
> >>> -m 2048 \
> >>> -machine pseries \
> >>> -initrd 1.cpio \
> >>> -kernel vml312_virtio_net_dbg \
> >>> -nographic \
> >>> -vga none \
> >>> -netdev
> >>> tap,id=id3,ifname=tap-id3,script=ifup.sh,downscript=ifdown.sh,vhost=on \
> >>> -device virtio-net-pci,id=id4,netdev=id3,mac=C0:41:49:4b:00:00
> >>>
> >>>
> >>> That is bridge config:
> >>> [aik@dyn232 ~]$ brctl show
> >>> bridge name	bridge id		STP enabled	interfaces
> >>> brtest		8000.00145e992e88	no	pin	eth4
> >>>
> >>>
> >>> The ifup.sh script:
> >>> ifconfig $1 hw ether ee:01:02:03:04:05
> >>> /sbin/ifconfig $1 up
> >>> /usr/sbin/brctl addif brtest $1
> > 
> > 
> 
> 
> -- 
> Alexey