Re: [PATCH v3 17/19] vhost-user: iommu: postpone device creation until ring are mapped

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

On Fri, Nov 03, 2017 at 09:25:58AM +0100, Maxime Coquelin wrote:
> 
> 
> On 11/02/2017 05:02 PM, Maxime Coquelin wrote:
> > 
> > 
> > On 11/02/2017 09:21 AM, Maxime Coquelin wrote:
> > > Hi Lei,
> > > 
> > > On 11/02/2017 08:21 AM, Yao, Lei A wrote:
> > > > 
> > > ...
> > > > Hi, Maxime > I met one issue with your patch set during the v17.11 test.
> > > 
> > > Is it with v17.11-rc2 or -rc1?
> > > 
> > > > The test scenario is following,
> > > > 1.    Bind one NIC, use test-pmd set vhost-user with 2 queue
> > > > usertools/dpdk-devbind.py --bind=igb_uio 0000:05:00.0
> > > > ./x86_64-native-linuxapp-gcc/app/testpmd -c 0xe -n 4
> > > > --socket-mem 1024,1024 \
> > > > --vdev 'net_vhost0,iface=vhost-net,queues=2' - -i --rxq=2
> > > > --txq=2 --nb-cores=2 --rss-ip
> > > > 2.    Launch qemu with  virtio device which has 2 queue
> > > > 3.    In VM, launch testpmd with virtio-pmd using only 1 queue.
> > > > x86_64-native-linuxapp-gcc/app/testpmd -c 0x07 -n 3 - -i
> > > > --txqflags=0xf01 \
> > > > --rxq=1 --txq=1 --rss-ip --nb-cores=1
> > > > 
> > > > First,
> > > > commit 09927b5249694bad1c094d3068124673722e6b8f
> > > > vhost: translate ring addresses when IOMMU enabled
> > > > The patch causes no traffic in PVP test. but link status is
> > > > still up in vhost-user.
> > > > 
> > > > Second,
> > > > eefac9536a901a1f0bb52aa3b6fec8f375f09190
> > > > vhost: postpone device creation until rings are mapped
> > > > The patch causes link status "down" in vhost-user.
> > 
> > I reproduced this one, and understand why link status remains down.
> > My series did fixed a potential issue Michael raised, that the vring
> > addresses should only interpreted once the ring is enabled.
> > When VHOST_USER_F_PROTOCOL_FEATURES is negotiated, the rings addrs are
> > translated when ring is enabled via VHOST_USER_SET_VRING_ENABLE.
> > When not negotiated, the ring is considered started enabled, so
> > translation is done at VHOST_USER_SET_VRING_KICK time.
> > 
> > In your case, protocol features are negotiated, so the ring addresses
> > are translated at enable time. The problem is that the code considers
> > the device is ready once addresses for all the rings are translated.
> > But since only the first pair of rings is used, it never happens, and
> > the link remains down.
> > 
> > One of the reason this check is done is to avoid starting the PMD
> > threads before the addresses are translated in case of NUMA
> > reallocation, as virtqueues and virtio-net device structs can be
> > reallocated on a different node.
> > 
> > I think the right fix would be to only perform NUMA reallocation for
> > vring 0, as today we would end-up reallocating virtio-net struct
> > mulitple time if VQs are on different NUMA nodes.
> > 
> > Doing that, we could then consider the device is ready if vring 0 is
> > enabled and its ring addresses are translated, and if other vrings have
> > been kicked.
> > 
> > I'll post a patch shortly implementing this idea.
> 
> The proposed solution doesn't work, because disabled queues get accessed at
> device start time:
> 
> int
> rte_vhost_enable_guest_notification(int vid, uint16_t queue_id, int enable)
> {
> ..
> 	dev->virtqueue[queue_id]->used->flags = VRING_USED_F_NO_NOTIFY;
> 	return 0;
> }
> 
> The above function being called in Vhost PMD for every queues, enabled
> or not. While we could fix the PMD, it could break other applications
> using the Vhost lib API directly, so we cannot translate at enable
> time reliably.
> 
> I think we may be a bit less conservative, and postpone addresses
> translation at kick time, whatever VHOST_USER_F_PROTOCOL_FEATURES is
> negotiated or not.
> 
> Regards,
> Maxime
> 
> > Thanks,
> > Maxime

I agree, enabling has nothing to do with it.

The spec is quite explicit:

Client must only process each ring when it is started.

and

Client must start ring upon receiving a kick (that is, detecting that file
descriptor is readable) on the descriptor specified by
VHOST_USER_SET_VRING_KICK, and stop ring upon receiving
VHOST_USER_GET_VRING_BASE.

-- 
MST