[dpdk-dev] [PATCH] vhost: stop device before updating public vring data

[Maxime Coquelin]

On 03/07/2018 11:59 AM, Wodkowski, PawelX wrote:
>> -----Original Message-----
>> From: Maxime Coquelin [mailto:maxime.coquelin at redhat.com]
>> Sent: Wednesday, March 7, 2018 11:09 AM
>> To: Wodkowski, PawelX <pawelx.wodkowski at intel.com>; Kulasek, TomaszX
>> <tomaszx.kulasek at intel.com>; yliu at fridaylinux.org
>> Cc: Verkamp, Daniel <daniel.verkamp at intel.com>; Harris, James R
>> <james.r.harris at intel.com>; dev at dpdk.org; Stojaczyk, DariuszX
>> <dariuszx.stojaczyk at intel.com>
>> Subject: Re: [dpdk-dev] [PATCH] vhost: stop device before updating public vring
>> data
>>
>>
>>
>> On 03/07/2018 10:16 AM, Wodkowski, PawelX wrote:
>>>> -----Original Message-----
>>>> From: Maxime Coquelin [mailto:maxime.coquelin at redhat.com]
>>>> Sent: Tuesday, March 6, 2018 5:27 PM
>>>> To: Kulasek, TomaszX <tomaszx.kulasek at intel.com>; yliu at fridaylinux.org
>>>> Cc: Verkamp, Daniel <daniel.verkamp at intel.com>; Harris, James R
>>>> <james.r.harris at intel.com>; Wodkowski, PawelX
>>>> <pawelx.wodkowski at intel.com>; dev at dpdk.org; Stojaczyk, DariuszX
>>>> <dariuszx.stojaczyk at intel.com>
>>>> Subject: Re: [dpdk-dev] [PATCH] vhost: stop device before updating public
>> vring
>>>> data
>>>>
>>>> Hi Tomasz,
>>>>
>>>> On 03/05/2018 05:11 PM, Tomasz Kulasek wrote:
>>>>> For now DPDK assumes that callfd, kickfd and last_idx are being set just
>>>>> once during vring initialization and device cannot be running while DPDK
>>>>> receives SET_VRING_KICK, SET_VRING_CALL and SET_VRING_BASE
>> messages.
>>>>> However, that assumption is wrong. For Vhost SCSI messages might arrive
>>>>> at any point of time, possibly multiple times, one after another.
>>>>>
>>>>> QEMU issues SET_VRING_CALL once during device initialization, then again
>>>>> during device start. The second message will close previous callfd,
>>>>> which is still being used by the user-implementation of vhost device.
>>>>> This results in writing to invalid (closed) callfd.
>>>>>
>>>>> Other messages like SET_FEATURES, SET_VRING_ADDR etc also will
>> change
>>>>> internal state of VQ or device. To prevent race condition device should
>>>>> also be stopped before updateing vring data.
>>>>>
>>>>> Signed-off-by: Dariusz Stojaczyk<dariuszx.stojaczyk at intel.com>
>>>>> Signed-off-by: Pawel Wodkowski<pawelx.wodkowski at intel.com>
>>>>> Signed-off-by: Tomasz Kulasek<tomaszx.kulasek at intel.com>
>>>>> ---
>>>>>     lib/librte_vhost/vhost_user.c | 40
>>>> ++++++++++++++++++++++++++++++++++++++++
>>>>>     1 file changed, 40 insertions(+)
>>>>
>>>> In last release, we have introduced a per-virtqueue lock to protect
>>>> vring handling against asynchronous device changes.
>>>>
>>>> I think that would solve the issue you are facing, but you would need
>>>> to export the VQs locking functions to the vhost-user lib API to be
>>>> able to use it.
>>>>
>>>> I don't think your current patch is the right solution anyway, because
>>>> it destroys the device in case we don't want it to remain alive, like
>>>> set_log_base, or set_features when only the logging feature gets
>>>> enabled.
>>>
>>> Please correct me if I can't see something obvious, but how this lock protect
>> against eg
>>> SET_MEM_TABLE message? Current flow you are thinking of is:
>>>
>>> DPDK vhost-user thread
>>> 1.1. vhost_user_lock_all_queue_pairs()
>>> 1.2. vhost_user_set_mem_table()
>>> 1.3. vhost_user_unlock_all_queue_pairs()
>>>
>>> BACKEND: virito-net:
>>> 2.1. rte_spinlock_lock(&vq->access_lock);
>>> 2.2. Process vrings and copy all data
>>> 2.3. rte_spinlock_unlock(&vq->access_lock);
>>>
>>> Yes, it will synchronize access to virtio_net structure but what if the BACKEND
>> is in
>>> zero copy mode and/or pass buffers to physical device? The request will
>>> not end in 2.2 and you unmap the memory regions in the middle of request.
>>> Even worse, the physical device will just abort the request but BACKEND can
>> segfault
>>> or write random memory because BACKEND try to use invalid memory
>> address
>>> (retrieved at request start).
>>
>> Right, it doesn't work with zero-copy.
>>
>>> To use this per-virtqueue lock:
>>> 1. the lock need to be held from request start to the end - but this can starve
>> DPDK
>>> vhost-user thread as there might be many request on-the-fly and when one is
>> done
>>> the new one might be started.
>>> 2. Becouse we don't know if something changed between requst start and
>> request end
>>> BACKEND need walk through all descriptors chain at the request end and do
>> the
>>> rte_vhost_gpa_to_vva() again.
>>>
>>> The SET_MEM_TABLE is most obvious message but the same is true for other
>> like
>>> VHOST_IOTLB_INVALIDATE or SET_FEATURES.
>>
>> SET_FEATURE should never be sent as soon as the device is started,
>> except to enable logging.
>>
>> For VHOST_IOTLB_INVALIDATE, the solution might be to have a ref counter
>> per entry, and to only remove it for the cache once it is zero and send
>> the reply-ack tothe master once this is done. But the cost would be huge
>> as with large entries, a lot of threads might increment/decrement the
>> same variable so there will be contention.
>>
>> For all other cases, like SET_MEM_TABLE, maybe the solution is to
>> disable/enable all the queues using the existing ops.
>> The application or library would have to take care that no guest buffers
>> are in the wild before returning from the disable.
>>
>> Do you think that would work?
> 
> What kind of ops can be used to reliably disable all queues and inform backend what
> changed beside new_device/destroy_device? Those informations are very well hidden
> inside vhost.c and vhost-user.c files.

(struct vhost_device_ops).vring_state_changed()

When a queue is disabled, I think we can expect the application won't
use its resources anymore.

> 
> I think we need new set of ops/callbacks in vhost_device_ops struct that let the backend
> decide how to behave in response on certain messages like SET_MEM_TABLE.

Yes, we can think of extending the API with new ops if
vring_state_change is not enough.

> Also I would propose to revert the a3688046995f "vhost: protect active rings from
> async ring changes" and replace it by behavior of this patch. I see now that this patch
> need to handle other cases like VHOST_USER_IOTLB_MSG but until we have new set of
> callbacks I think this is only reliable and fast solution.

Nack, we won't destroy the device every time we receive a vhost-user
request.

And for reverting the patch you propose, it won't happen as long as we
don't have a viable alternative, and as long as the applications like
OVS won't use it.

This patch fixes real issues faced by the user, like live migration with
multiple queues or memory hotplug. The problem is that it does not
take zero-copy case into account.

Maxime

> Pawel
> 
>>
>> Cheers,
>> Maxime
>>
>>> Pawel
>>>
>>>>
>>>> Cheers,
>>>> Maxime