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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 11/03/2017 04:15 PM, Michael S. Tsirkin wrote:
> 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.
> 

Thanks for the confirmation Michael, fix posted.

Maxime