Re: [PATCH v2 1/5] virtio-mmio: add notify feature for per-queue

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

On Mon, Feb 10, 2020 at 05:05:17PM +0800, Zha Bin wrote:
> From: Liu Jiang <gerry@linux.alibaba.com>
> 
> The standard virtio-mmio devices use notification register to signal
> backend. This will cause vmexits and slow down the performance when we
> passthrough the virtio-mmio devices to guest virtual machines.
> We proposed to update virtio over MMIO spec to add the per-queue
> notify feature VIRTIO_F_MMIO_NOTIFICATION[1]. It can allow the VMM to
> configure notify location for each queue.
> 
> [1] https://lkml.org/lkml/2020/1/21/31
> 
> Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>
> Co-developed-by: Zha Bin <zhabin@linux.alibaba.com>
> Signed-off-by: Zha Bin <zhabin@linux.alibaba.com>
> Co-developed-by: Jing Liu <jing2.liu@linux.intel.com>
> Signed-off-by: Jing Liu <jing2.liu@linux.intel.com>
> Co-developed-by: Chao Peng <chao.p.peng@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>


Hmm. Any way to make this static so we don't need
base and multiplier?

> ---
>  drivers/virtio/virtio_mmio.c       | 37 +++++++++++++++++++++++++++++++++++--
>  include/uapi/linux/virtio_config.h |  8 +++++++-
>  2 files changed, 42 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
> index 97d5725..1733ab97 100644
> --- a/drivers/virtio/virtio_mmio.c
> +++ b/drivers/virtio/virtio_mmio.c
> @@ -90,6 +90,9 @@ struct virtio_mmio_device {
>  	/* a list of queues so we can dispatch IRQs */
>  	spinlock_t lock;
>  	struct list_head virtqueues;
> +
> +	unsigned short notify_base;
> +	unsigned short notify_multiplier;
>  };
>  
>  struct virtio_mmio_vq_info {
> @@ -98,6 +101,9 @@ struct virtio_mmio_vq_info {
>  
>  	/* the list node for the virtqueues list */
>  	struct list_head node;
> +
> +	/* Notify Address*/
> +	unsigned int notify_addr;
>  };
>  
>  
> @@ -119,13 +125,23 @@ static u64 vm_get_features(struct virtio_device *vdev)
>  	return features;
>  }
>  
> +static void vm_transport_features(struct virtio_device *vdev, u64 features)
> +{
> +	if (features & BIT_ULL(VIRTIO_F_MMIO_NOTIFICATION))
> +		__virtio_set_bit(vdev, VIRTIO_F_MMIO_NOTIFICATION);
> +}
> +
>  static int vm_finalize_features(struct virtio_device *vdev)
>  {
>  	struct virtio_mmio_device *vm_dev = to_virtio_mmio_device(vdev);
> +	u64 features = vdev->features;
>  
>  	/* Give virtio_ring a chance to accept features. */
>  	vring_transport_features(vdev);
>  
> +	/* Give virtio_mmio a chance to accept features. */
> +	vm_transport_features(vdev, features);
> +
>  	/* Make sure there is are no mixed devices */
>  	if (vm_dev->version == 2 &&
>  			!__virtio_test_bit(vdev, VIRTIO_F_VERSION_1)) {
> @@ -272,10 +288,13 @@ static void vm_reset(struct virtio_device *vdev)
>  static bool vm_notify(struct virtqueue *vq)
>  {
>  	struct virtio_mmio_device *vm_dev = to_virtio_mmio_device(vq->vdev);
> +	struct virtio_mmio_vq_info *info = vq->priv;
>  
> -	/* We write the queue's selector into the notification register to
> +	/* We write the queue's selector into the Notify Address to
>  	 * signal the other end */
> -	writel(vq->index, vm_dev->base + VIRTIO_MMIO_QUEUE_NOTIFY);
> +	if (info)
> +		writel(vq->index, vm_dev->base + info->notify_addr);
> +
>  	return true;
>  }
>  
> @@ -434,6 +453,12 @@ static struct virtqueue *vm_setup_vq(struct virtio_device *vdev, unsigned index,
>  	vq->priv = info;
>  	info->vq = vq;
>  
> +	if (__virtio_test_bit(vdev, VIRTIO_F_MMIO_NOTIFICATION))
> +		info->notify_addr = vm_dev->notify_base +
> +				vm_dev->notify_multiplier * vq->index;
> +	else
> +		info->notify_addr = VIRTIO_MMIO_QUEUE_NOTIFY;
> +
>  	spin_lock_irqsave(&vm_dev->lock, flags);
>  	list_add(&info->node, &vm_dev->virtqueues);
>  	spin_unlock_irqrestore(&vm_dev->lock, flags);
> @@ -471,6 +496,14 @@ static int vm_find_vqs(struct virtio_device *vdev, unsigned nvqs,
>  		return irq;
>  	}
>  
> +	if (__virtio_test_bit(vdev, VIRTIO_F_MMIO_NOTIFICATION)) {
> +		unsigned int notify = readl(vm_dev->base +
> +				VIRTIO_MMIO_QUEUE_NOTIFY);


that register is documented as:

/* Queue notifier - Write Only */
#define VIRTIO_MMIO_QUEUE_NOTIFY        0x050

so at least you need to update the doc.

> +
> +		vm_dev->notify_base = notify & 0xffff;
> +		vm_dev->notify_multiplier = (notify >> 16) & 0xffff;

are 16 bit base/limit always enough?
In fact won't we be short on 16 bit address space
in a rather short order if queues use up a page
of space at a time?


> +	}
> +
>  	err = request_irq(irq, vm_interrupt, IRQF_SHARED,
>  			dev_name(&vdev->dev), vm_dev);
>  	if (err)
> diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h
> index ff8e7dc..5d93c01 100644
> --- a/include/uapi/linux/virtio_config.h
> +++ b/include/uapi/linux/virtio_config.h
> @@ -52,7 +52,7 @@
>   * rest are per-device feature bits.
>   */
>  #define VIRTIO_TRANSPORT_F_START	28
> -#define VIRTIO_TRANSPORT_F_END		38
> +#define VIRTIO_TRANSPORT_F_END		40
>  
>  #ifndef VIRTIO_CONFIG_NO_LEGACY
>  /* Do we get callbacks when the ring is completely used, even if we've
> @@ -88,4 +88,10 @@
>   * Does the device support Single Root I/O Virtualization?
>   */
>  #define VIRTIO_F_SR_IOV			37
> +
> +/*
> + * This feature indicates the enhanced notification support on MMIO transport
> + * layer.

Let's replace this with an actual description of the enhancement please
otherwise it will not make sense in a couple of months.

e.g. "Per queue notification address"?


> + */
> +#define VIRTIO_F_MMIO_NOTIFICATION	39
>  #endif /* _UAPI_LINUX_VIRTIO_CONFIG_H */
> -- 
> 1.8.3.1