linux-kernel - Re: [PATCH V2 3/5] virtio: introduce config op to synchronize vring callbacks

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Date:   Thu, 7 Apr 2022 14:25:44 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, maz@...nel.org, tglx@...utronix.de,
        peterz@...radead.org, sgarzare@...hat.com,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH V2 3/5] virtio: introduce config op to synchronize vring
 callbacks


在 2022/4/6 下午7:59, Michael S. Tsirkin 写道:
> On Wed, Apr 06, 2022 at 04:35:36PM +0800, Jason Wang wrote:
>> This patch introduce
> introduces
>
>> a new
> new
>
>> virtio config ops to vring
>> callbacks. Transport specific method is required to call
>> synchornize_irq() on the IRQs. For the transport that doesn't provide
>> synchronize_vqs(), use synchornize_rcu() as a fallback.
>>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: "Paul E. McKenney" <paulmck@...nel.org>
>> Cc: Marc Zyngier <maz@...nel.org>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>>   include/linux/virtio_config.h | 16 ++++++++++++++++
>>   1 file changed, 16 insertions(+)
>>
>> diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
>> index b341dd62aa4d..08b73d9bbff2 100644
>> --- a/include/linux/virtio_config.h
>> +++ b/include/linux/virtio_config.h
>> @@ -57,6 +57,8 @@ struct virtio_shm_region {
>>    *		include a NULL entry for vqs unused by driver
>>    *	Returns 0 on success or error status
>>    * @del_vqs: free virtqueues found by find_vqs().
>> + * @synchronize_vqs: synchronize with the virtqueue callbacks.
>> + *	vdev: the virtio_device
> I think I prefer synchronize_callbacks


Ok, I will rename it.


>
>>    * @get_features: get the array of feature bits for this device.
>>    *	vdev: the virtio_device
>>    *	Returns the first 64 feature bits (all we currently need).
>> @@ -89,6 +91,7 @@ struct virtio_config_ops {
>>   			const char * const names[], const bool *ctx,
>>   			struct irq_affinity *desc);
>>   	void (*del_vqs)(struct virtio_device *);
>> +	void (*synchronize_vqs)(struct virtio_device *);
>>   	u64 (*get_features)(struct virtio_device *vdev);
>>   	int (*finalize_features)(struct virtio_device *vdev);
>>   	const char *(*bus_name)(struct virtio_device *vdev);
>> @@ -217,6 +220,19 @@ int virtio_find_vqs_ctx(struct virtio_device *vdev, unsigned nvqs,
>>   				      desc);
>>   }
>>   
>> +/**
>> + * virtio_synchronize_vqs - synchronize with virtqueue callbacks
>> + * @vdev: the device
>> + */
>> +static inline
>> +void virtio_synchronize_vqs(struct virtio_device *dev)
>> +{
>> +	if (dev->config->synchronize_vqs)
>> +		dev->config->synchronize_vqs(dev);
>> +	else
>> +		synchronize_rcu();
> I am not sure about this fallback and the latency impact.


Unless each transport can implement their own synchronization routine, 
we need something that can do best effort as fallback here.


> Maybe synchronize_rcu_expedited is better here.


Not sure, it might lead IPIs and according to the 
Documentation/RCU/checklist.rst:


"""

         The expedited forms of these primitives have the same semantics
         as the non-expedited forms, but expediting is both expensive and
         (with the exception of synchronize_srcu_expedited()) unfriendly
         to real-time workloads.  Use of the expedited primitives should
         be restricted to rare configuration-change operations that would
         not normally be undertaken while a real-time workload is running.
         However, real-time workloads can use rcupdate.rcu_normal kernel
         boot parameter to completely disable expedited grace periods,
         though this might have performance implications.

"""

It will be expensive for real time workloads.

Thanks


>
>> +}
>> +
>>   /**
>>    * virtio_device_ready - enable vq use in probe function
>>    * @vdev: the device
>> -- 
>> 2.25.1