Re: [PATCH V6 3/9] virtio: introduce config op to synchronize vring callbacks

[Xuan Zhuo]

On Fri, 27 May 2022 14:01:14 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> This patch introduces new virtio config op to vring
> callbacks. Transport specific method is required to make sure the
> write before this function is visible to the vring_interrupt() that is
> called after the return of this function. For the transport that
> doesn't provide synchronize_vqs(), use synchornize_rcu() which
> synchronize with IRQ implicitly as a fallback.
>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Halil Pasic <pasic@xxxxxxxxxxxxx>
> Cc: Cornelia Huck <cohuck@xxxxxxxxxx>
> Cc: Vineeth Vijayan <vneethv@xxxxxxxxxxxxx>
> Cc: Peter Oberparleiter <oberpar@xxxxxxxxxxxxx>
> Cc: linux-s390@xxxxxxxxxxxxxxx
> Reviewed-by: Cornelia Huck <cohuck@xxxxxxxxxx>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>

Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>

> ---
>  include/linux/virtio_config.h | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> index b341dd62aa4d..25be018810a7 100644
> --- a/include/linux/virtio_config.h
> +++ b/include/linux/virtio_config.h
> @@ -57,6 +57,11 @@ 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_cbs: synchronize with the virtqueue callbacks (optional)
> + *      The function guarantees that all memory operations on the
> + *      queue before it are visible to the vring_interrupt() that is
> + *      called after it.
> + *      vdev: the virtio_device
>   * @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 +94,7 @@ struct virtio_config_ops {
>  			const char * const names[], const bool *ctx,
>  			struct irq_affinity *desc);
>  	void (*del_vqs)(struct virtio_device *);
> +	void (*synchronize_cbs)(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 +223,25 @@ int virtio_find_vqs_ctx(struct virtio_device *vdev, unsigned nvqs,
>  				      desc);
>  }
>
> +/**
> + * virtio_synchronize_cbs - synchronize with virtqueue callbacks
> + * @vdev: the device
> + */
> +static inline
> +void virtio_synchronize_cbs(struct virtio_device *dev)
> +{
> +	if (dev->config->synchronize_cbs) {
> +		dev->config->synchronize_cbs(dev);
> +	} else {
> +		/*
> +		 * A best effort fallback to synchronize with
> +		 * interrupts, preemption and softirq disabled
> +		 * regions. See comment above synchronize_rcu().
> +		 */
> +		synchronize_rcu();
> +	}
> +}
> +
>  /**
>   * virtio_device_ready - enable vq use in probe function
>   * @vdev: the device
> --
> 2.25.1
>