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

Jason Wang
2017-09-27T00:38:24Z

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

>
>> --
>> 2.7.4