Re: [PATCH net-next RFC 3/5] vhost: introduce vhost_add_used_idx()

[Jason Wang <jasowang@redhat.com>]

On 2017年09月28日 06:58, Michael S. Tsirkin wrote:
> On Wed, Sep 27, 2017 at 08:38:24AM +0800, Jason Wang wrote:
>> On 2017年09月27日 03:13, Michael S. Tsirkin wrote:
>>> On Fri, Sep 22, 2017 at 04:02:33PM +0800, Jason Wang wrote:
>>>> This patch introduces a helper which just increase the used idx. This
>>>> will be used in pair with vhost_prefetch_desc_indices() by batching
>>>> code.
>>>>
>>>> Signed-off-by: Jason Wang<jasowang@redhat.com>
>>>> ---
>>>>    drivers/vhost/vhost.c | 33 +++++++++++++++++++++++++++++++++
>>>>    drivers/vhost/vhost.h |  1 +
>>>>    2 files changed, 34 insertions(+)
>>>>
>>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>>> index 8424166d..6532cda 100644
>>>> --- a/drivers/vhost/vhost.c
>>>> +++ b/drivers/vhost/vhost.c
>>>> @@ -2178,6 +2178,39 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
>>>>    }
>>>>    EXPORT_SYMBOL_GPL(vhost_add_used);
>>>> +int vhost_add_used_idx(struct vhost_virtqueue *vq, int n)
>>>> +{
>>>> +	u16 old, new;
>>>> +
>>>> +	old = vq->last_used_idx;
>>>> +	new = (vq->last_used_idx += n);
>>>> +	/* If the driver never bothers to signal in a very long while,
>>>> +	 * used index might wrap around. If that happens, invalidate
>>>> +	 * signalled_used index we stored. TODO: make sure driver
>>>> +	 * signals at least once in 2^16  and remove this.
>>>> +	 */
>>>> +	if (unlikely((u16)(new - vq->signalled_used) < (u16)(new - old)))
>>>> +		vq->signalled_used_valid = false;
>>>> +
>>>> +	/* Make sure buffer is written before we update index. */
>>>> +	smp_wmb();
>>>> +	if (vhost_put_user(vq, cpu_to_vhost16(vq, vq->last_used_idx),
>>>> +			   &vq->used->idx)) {
>>>> +		vq_err(vq, "Failed to increment used idx");
>>>> +		return -EFAULT;
>>>> +	}
>>>> +	if (unlikely(vq->log_used)) {
>>>> +		/* Log used index update. */
>>>> +		log_write(vq->log_base,
>>>> +			  vq->log_addr + offsetof(struct vring_used, idx),
>>>> +			  sizeof(vq->used->idx));
>>>> +		if (vq->log_ctx)
>>>> +			eventfd_signal(vq->log_ctx, 1);
>>>> +	}
>>>> +	return 0;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(vhost_add_used_idx);
>>>> +
>>>>    static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>>>>    			    struct vring_used_elem *heads,
>>>>    			    unsigned count)
>>>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>>>> index 16c2cb6..5dd6c05 100644
>>>> --- a/drivers/vhost/vhost.h
>>>> +++ b/drivers/vhost/vhost.h
>>>> @@ -199,6 +199,7 @@ int __vhost_get_vq_desc(struct vhost_virtqueue *vq,
>>>>    void vhost_discard_vq_desc(struct vhost_virtqueue *, int n);
>>>>    int vhost_vq_init_access(struct vhost_virtqueue *);
>>>> +int vhost_add_used_idx(struct vhost_virtqueue *vq, int n);
>>>>    int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
>>>>    int vhost_add_used_n(struct vhost_virtqueue *, struct vring_used_elem *heads,
>>>>    		     unsigned count);
>>> Please change the API to hide the fact that there's an index that needs
>>> to be updated.
>> In fact, an interesting optimization on top is just call
>> vhost_add_used_idx(vq, n) instead of n vhost_add_used_idx(vq, 1). That's the
>> reason I leave n in the API.
>>
>> Thanks
> Right but you could increment some internal counter in the vq
> structure then update the used index using some api
> with a generic name, e.g.  add_used_complete or something like this.
>

Right, I see.

Thanks