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

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

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@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Jason Wang <jasowang@redhat.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

>   * @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.
Maybe synchronize_rcu_expedited is better here.

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

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