[SRU,Zesty,1/1] Revert "vhost: cache used event for better performance"

BugLink: http://bugs.launchpad.net/bugs/1711251

This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92.
---
 drivers/vhost/vhost.c | 28 ++++++----------------------
 drivers/vhost/vhost.h |  3 ---
 2 files changed, 6 insertions(+), 25 deletions(-)

On Fri, Aug 25, 2017 at 12:07:55PM -0400, Joseph Salisbury wrote:
> BugLink: http://bugs.launchpad.net/bugs/1711251
> 
> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92.

I'm curious why you reverted directly instead of cherry picking the
upstream revert. In either case the patch needs your sign-off, which is
missing here.

On 08/28/2017 08:54 AM, Seth Forshee wrote:
> On Fri, Aug 25, 2017 at 12:07:55PM -0400, Joseph Salisbury wrote:
>> BugLink: http://bugs.launchpad.net/bugs/1711251
>>
>> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92.
> I'm curious why you reverted directly instead of cherry picking the
> upstream revert. In either case the patch needs your sign-off, which is
> missing here.

Cherry picking the revert from upstream was not clean and required a
backport.  However, If I did the revert directly to Zesty it was clean. 
Do you think it would be better to cherry pick from upstream and back port?

On Mon, Aug 28, 2017 at 09:14:28AM -0400, Joseph Salisbury wrote:
> On 08/28/2017 08:54 AM, Seth Forshee wrote:
> > On Fri, Aug 25, 2017 at 12:07:55PM -0400, Joseph Salisbury wrote:
> >> BugLink: http://bugs.launchpad.net/bugs/1711251
> >>
> >> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92.
> > I'm curious why you reverted directly instead of cherry picking the
> > upstream revert. In either case the patch needs your sign-off, which is
> > missing here.
> 
> Cherry picking the revert from upstream was not clean and required a
> backport.  However, If I did the revert directly to Zesty it was clean. 
> Do you think it would be better to cherry pick from upstream and back port?

No, that's a good enough reason. However I think it probably does need
to be marked as sauce in that case, and I think it would be useful to
mention the upstream commit and the reason for not cherry picking it in
the patch description.

On 08/28/2017 10:05 AM, Seth Forshee wrote:
> On Mon, Aug 28, 2017 at 09:14:28AM -0400, Joseph Salisbury wrote:
>> On 08/28/2017 08:54 AM, Seth Forshee wrote:
>>> On Fri, Aug 25, 2017 at 12:07:55PM -0400, Joseph Salisbury wrote:
>>>> BugLink: http://bugs.launchpad.net/bugs/1711251
>>>>
>>>> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92.
>>> I'm curious why you reverted directly instead of cherry picking the
>>> upstream revert. In either case the patch needs your sign-off, which is
>>> missing here.
>> Cherry picking the revert from upstream was not clean and required a
>> backport.  However, If I did the revert directly to Zesty it was clean. 
>> Do you think it would be better to cherry pick from upstream and back port?
> No, that's a good enough reason. However I think it probably does need
> to be marked as sauce in that case, and I think it would be useful to
> mention the upstream commit and the reason for not cherry picking it in
> the patch description.

Thanks for the feedback, Seth.  I'll NAK this one and send a v2.

On 01/09/17 17:23, Joseph Salisbury wrote:
> BugLink: http://bugs.launchpad.net/bugs/1711251
> 
> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92.
> ---
>  drivers/vhost/vhost.c | 28 ++++++----------------------
>  drivers/vhost/vhost.h |  3 ---
>  2 files changed, 6 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 8f99fe0..8d05a40 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -290,7 +290,6 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  	vq->avail = NULL;
>  	vq->used = NULL;
>  	vq->last_avail_idx = 0;
> -	vq->last_used_event = 0;
>  	vq->avail_idx = 0;
>  	vq->last_used_idx = 0;
>  	vq->signalled_used = 0;
> @@ -1325,7 +1324,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
>  			r = -EINVAL;
>  			break;
>  		}
> -		vq->last_avail_idx = vq->last_used_event = s.num;
> +		vq->last_avail_idx = s.num;
>  		/* Forget the cached index value. */
>  		vq->avail_idx = vq->last_avail_idx;
>  		break;
> @@ -2157,6 +2156,10 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  	__u16 old, new;
>  	__virtio16 event;
>  	bool v;
> +	/* Flush out used index updates. This is paired
> +	 * with the barrier that the Guest executes when enabling
> +	 * interrupts. */
> +	smp_mb();
>  
>  	if (vhost_has_feature(vq, VIRTIO_F_NOTIFY_ON_EMPTY) &&
>  	    unlikely(vq->avail_idx == vq->last_avail_idx))
> @@ -2164,10 +2167,6 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  
>  	if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
>  		__virtio16 flags;
> -		/* Flush out used index updates. This is paired
> -		 * with the barrier that the Guest executes when enabling
> -		 * interrupts. */
> -		smp_mb();
>  		if (vhost_get_user(vq, flags, &vq->avail->flags)) {
>  			vq_err(vq, "Failed to get flags");
>  			return true;
> @@ -2182,26 +2181,11 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  	if (unlikely(!v))
>  		return true;
>  
> -	/* We're sure if the following conditions are met, there's no
> -	 * need to notify guest:
> -	 * 1) cached used event is ahead of new
> -	 * 2) old to new updating does not cross cached used event. */
> -	if (vring_need_event(vq->last_used_event, new + vq->num, new) &&
> -	    !vring_need_event(vq->last_used_event, new, old))
> -		return false;
> -
> -	/* Flush out used index updates. This is paired
> -	 * with the barrier that the Guest executes when enabling
> -	 * interrupts. */
> -	smp_mb();
> -
>  	if (vhost_get_user(vq, event, vhost_used_event(vq))) {
>  		vq_err(vq, "Failed to get used event idx");
>  		return true;
>  	}
> -	vq->last_used_event = vhost16_to_cpu(vq, event);
> -
> -	return vring_need_event(vq->last_used_event, new, old);
> +	return vring_need_event(vhost16_to_cpu(vq, event), new, old);
>  }
>  
>  /* This actually signals the guest, using eventfd. */
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index a9cbbb1..78f3c5f 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -107,9 +107,6 @@ struct vhost_virtqueue {
>  	/* Last index we used. */
>  	u16 last_used_idx;
>  
> -	/* Last used evet we've seen */
> -	u16 last_used_event;
> -
>  	/* Used flags */
>  	u16 used_flags;
>  
> 
Seems very reasonable to me. Thanks Joe

Acked-by: Colin Ian King <colin.king@canonical.com>

On 25.08.2017 18:07, Joseph Salisbury wrote:
> BugLink: http://bugs.launchpad.net/bugs/1711251
> 
commit 8d65843c44269c21e95c98090d9bb4848d473853 upstream

> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92.

Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>

> ---

I would add the upstream sha1 reference in upstream stable style. And you should
do reverts also with -s ;)

-Stefan

>  drivers/vhost/vhost.c | 28 ++++++----------------------
>  drivers/vhost/vhost.h |  3 ---
>  2 files changed, 6 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 8f99fe0..8d05a40 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -290,7 +290,6 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  	vq->avail = NULL;
>  	vq->used = NULL;
>  	vq->last_avail_idx = 0;
> -	vq->last_used_event = 0;
>  	vq->avail_idx = 0;
>  	vq->last_used_idx = 0;
>  	vq->signalled_used = 0;
> @@ -1325,7 +1324,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
>  			r = -EINVAL;
>  			break;
>  		}
> -		vq->last_avail_idx = vq->last_used_event = s.num;
> +		vq->last_avail_idx = s.num;
>  		/* Forget the cached index value. */
>  		vq->avail_idx = vq->last_avail_idx;
>  		break;
> @@ -2157,6 +2156,10 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  	__u16 old, new;
>  	__virtio16 event;
>  	bool v;
> +	/* Flush out used index updates. This is paired
> +	 * with the barrier that the Guest executes when enabling
> +	 * interrupts. */
> +	smp_mb();
>  
>  	if (vhost_has_feature(vq, VIRTIO_F_NOTIFY_ON_EMPTY) &&
>  	    unlikely(vq->avail_idx == vq->last_avail_idx))
> @@ -2164,10 +2167,6 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  
>  	if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
>  		__virtio16 flags;
> -		/* Flush out used index updates. This is paired
> -		 * with the barrier that the Guest executes when enabling
> -		 * interrupts. */
> -		smp_mb();
>  		if (vhost_get_user(vq, flags, &vq->avail->flags)) {
>  			vq_err(vq, "Failed to get flags");
>  			return true;
> @@ -2182,26 +2181,11 @@ static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  	if (unlikely(!v))
>  		return true;
>  
> -	/* We're sure if the following conditions are met, there's no
> -	 * need to notify guest:
> -	 * 1) cached used event is ahead of new
> -	 * 2) old to new updating does not cross cached used event. */
> -	if (vring_need_event(vq->last_used_event, new + vq->num, new) &&
> -	    !vring_need_event(vq->last_used_event, new, old))
> -		return false;
> -
> -	/* Flush out used index updates. This is paired
> -	 * with the barrier that the Guest executes when enabling
> -	 * interrupts. */
> -	smp_mb();
> -
>  	if (vhost_get_user(vq, event, vhost_used_event(vq))) {
>  		vq_err(vq, "Failed to get used event idx");
>  		return true;
>  	}
> -	vq->last_used_event = vhost16_to_cpu(vq, event);
> -
> -	return vring_need_event(vq->last_used_event, new, old);
> +	return vring_need_event(vhost16_to_cpu(vq, event), new, old);
>  }
>  
>  /* This actually signals the guest, using eventfd. */
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index a9cbbb1..78f3c5f 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -107,9 +107,6 @@ struct vhost_virtqueue {
>  	/* Last index we used. */
>  	u16 last_used_idx;
>  
> -	/* Last used evet we've seen */
> -	u16 last_used_event;
> -
>  	/* Used flags */
>  	u16 used_flags;
>  
>