Date:   Thu, 3 May 2018 10:09:49 +0800
From:   Tiwei Bie <tiwei.bie@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        wexu@...hat.com, jfreimann@...hat.com
Subject: Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring

On Thu, May 03, 2018 at 04:44:39AM +0300, Michael S. Tsirkin wrote:
> On Thu, May 03, 2018 at 09:11:16AM +0800, Tiwei Bie wrote:
> > On Wed, May 02, 2018 at 06:42:57PM +0300, Michael S. Tsirkin wrote:
> > > On Wed, May 02, 2018 at 11:12:55PM +0800, Tiwei Bie wrote:
> > > > On Wed, May 02, 2018 at 04:51:01PM +0300, Michael S. Tsirkin wrote:
> > > > > On Wed, May 02, 2018 at 03:28:19PM +0800, Tiwei Bie wrote:
> > > > > > On Wed, May 02, 2018 at 10:51:06AM +0800, Jason Wang wrote:
> > > > > > > On 2018年04月25日 13:15, Tiwei Bie wrote:
> > > > > > > > This commit introduces the event idx support in packed
> > > > > > > > ring. This feature is temporarily disabled, because the
> > > > > > > > implementation in this patch may not work as expected,
> > > > > > > > and some further discussions on the implementation are
> > > > > > > > needed, e.g. do we have to check the wrap counter when
> > > > > > > > checking whether a kick is needed?
> > > > > > > > 
> > > > > > > > Signed-off-by: Tiwei Bie <tiwei.bie@...el.com>
> > > > > > > > ---
> > > > > > > >   drivers/virtio/virtio_ring.c | 53 ++++++++++++++++++++++++++++++++++++++++----
> > > > > > > >   1 file changed, 49 insertions(+), 4 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > > > > > index 0181e93897be..b1039c2985b9 100644
> > > > > > > > --- a/drivers/virtio/virtio_ring.c
> > > > > > > > +++ b/drivers/virtio/virtio_ring.c
> > > > > > > > @@ -986,7 +986,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > > > > > > >   static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
> > > > > > > >   {
> > > > > > > >   	struct vring_virtqueue *vq = to_vvq(_vq);
> > > > > > > > -	u16 flags;
> > > > > > > > +	u16 new, old, off_wrap, flags;
> > > > > > > >   	bool needs_kick;
> > > > > > > >   	u32 snapshot;
> > > > > > > > @@ -995,7 +995,12 @@ static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
> > > > > > > >   	 * suppressions. */
> > > > > > > >   	virtio_mb(vq->weak_barriers);
> > > > > > > > +	old = vq->next_avail_idx - vq->num_added;
> > > > > > > > +	new = vq->next_avail_idx;
> > > > > > > > +	vq->num_added = 0;
> > > > > > > > +
> > > > > > > >   	snapshot = *(u32 *)vq->vring_packed.device;
> > > > > > > > +	off_wrap = virtio16_to_cpu(_vq->vdev, snapshot & 0xffff);
> > > > > > > >   	flags = cpu_to_virtio16(_vq->vdev, snapshot >> 16) & 0x3;
> > > > > > > >   #ifdef DEBUG
> > > > > > > > @@ -1006,7 +1011,10 @@ static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
> > > > > > > >   	vq->last_add_time_valid = false;
> > > > > > > >   #endif
> > > > > > > > -	needs_kick = (flags != VRING_EVENT_F_DISABLE);
> > > > > > > > +	if (flags == VRING_EVENT_F_DESC)
> > > > > > > > +		needs_kick = vring_need_event(off_wrap & ~(1<<15), new, old);
> > > > > > > 
> > > > > > > I wonder whether or not the math is correct. Both new and event are in the
> > > > > > > unit of descriptor ring size, but old looks not.
> > > > > > 
> > > > > > What vring_need_event() cares is the distance between
> > > > > > `new` and `old`, i.e. vq->num_added. So I think there
> > > > > > is nothing wrong with `old`. But the calculation of the
> > > > > > distance between `new` and `event_idx` isn't right when
> > > > > > `new` wraps. How do you think about the below code:
> > > > > > 
> > > > > > 	wrap_counter = off_wrap >> 15;
> > > > > > 	event_idx = off_wrap & ~(1<<15);
> > > > > > 	if (wrap_counter != vq->wrap_counter)
> > > > > > 		event_idx -= vq->vring_packed.num;
> > > > > > 	
> > > > > > 	needs_kick = vring_need_event(event_idx, new, old);
> > > > > 
> > > > > I suspect this hack won't work for non power of 2 ring.
> > > > 
> > > > Above code doesn't require the ring size to be a power of 2.
> > > > 
> > > > For (__u16)(new_idx - old), what we want to get is vq->num_added.
> > > > 
> > > > old = vq->next_avail_idx - vq->num_added;
> > > > new = vq->next_avail_idx;
> > > > 
> > > > When vq->next_avail_idx >= vq->num_added, it's obvious that,
> > > > (__u16)(new_idx - old) is vq->num_added.
> > > > 
> > > > And when vq->next_avail_idx < vq->num_added, new will be smaller
> > > > than old (old will be a big unsigned number), but (__u16)(new_idx
> > > > - old) is still vq->num_added.
> > > > 
> > > > For (__u16)(new_idx - event_idx - 1), when new wraps and event_idx
> > > > doesn't wrap, the most straightforward way to calculate it is:
> > > > (new + vq->vring_packed.num) - event_idx - 1.
> > > 
> > > So how about we use the straightforward way then?
> > 
> > You mean we do new += vq->vring_packed.num instead
> > of event_idx -= vq->vring_packed.num before calling
> > vring_need_event()?
> > 
> > The problem is that, the second param (new_idx) of
> > vring_need_event() will be used for:
> > 
> > (__u16)(new_idx - event_idx - 1)
> > (__u16)(new_idx - old)
> > 
> > So if we change new, we will need to change old too.
> 
> I think that since we have a branch there anyway,
> we are better off just special-casing if (wrap_counter != vq->wrap_counter).
> Treat is differenty and avoid casts.
> 
> > And that would be an ugly hack..
> > 
> > Best regards,
> > Tiwei Bie
> 
> I consider casts and huge numbers with two's complement
> games even uglier.

The dependency on two's complement game is introduced
since the split ring.

In packed ring, old is calculated via:

old = vq->next_avail_idx - vq->num_added;

In split ring, old is calculated via:

old = vq->avail_idx_shadow - vq->num_added;

In both cases, when vq->num_added is bigger, old will
be a big number.

Best regards,
Tiwei Bie

> 
> > > 
> > > > But we can also calculate it in this way:
> > > > 
> > > > event_idx -= vq->vring_packed.num;
> > > > (event_idx will be a big unsigned number)
> > > > 
> > > > Then (__u16)(new_idx - event_idx - 1) will be the value we want.
> > > > 
> > > > Best regards,
> > > > Tiwei Bie
> > > 
> > > 
> > > > > 
> > > > > 
> > > > > > Best regards,
> > > > > > Tiwei Bie
> > > > > > 
> > > > > > 
> > > > > > > 
> > > > > > > Thanks
> > > > > > > 
> > > > > > > > +	else
> > > > > > > > +		needs_kick = (flags != VRING_EVENT_F_DISABLE);
> > > > > > > >   	END_USE(vq);
> > > > > > > >   	return needs_kick;
> > > > > > > >   }
> > > > > > > > @@ -1116,6 +1124,15 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> > > > > > > >   	if (vq->last_used_idx >= vq->vring_packed.num)
> > > > > > > >   		vq->last_used_idx -= vq->vring_packed.num;
> > > > > > > > +	/* If we expect an interrupt for the next entry, tell host
> > > > > > > > +	 * by writing event index and flush out the write before
> > > > > > > > +	 * the read in the next get_buf call. */
> > > > > > > > +	if (vq->event_flags_shadow == VRING_EVENT_F_DESC)
> > > > > > > > +		virtio_store_mb(vq->weak_barriers,
> > > > > > > > +				&vq->vring_packed.driver->off_wrap,
> > > > > > > > +				cpu_to_virtio16(_vq->vdev, vq->last_used_idx |
> > > > > > > > +						(vq->wrap_counter << 15)));
> > > > > > > > +
> > > > > > > >   #ifdef DEBUG
> > > > > > > >   	vq->last_add_time_valid = false;
> > > > > > > >   #endif
> > > > > > > > @@ -1143,10 +1160,17 @@ static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
> > > > > > > >   	/* We optimistically turn back on interrupts, then check if there was
> > > > > > > >   	 * more to do. */
> > > > > > > > +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> > > > > > > > +	 * either clear the flags bit or point the event index at the next
> > > > > > > > +	 * entry. Always update the event index to keep code simple. */
> > > > > > > > +
> > > > > > > > +	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
> > > > > > > > +			vq->last_used_idx | (vq->wrap_counter << 15));
> > > > > > > >   	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> > > > > > > >   		virtio_wmb(vq->weak_barriers);
> > > > > > > > -		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> > > > > > > > +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
> > > > > > > > +						     VRING_EVENT_F_ENABLE;
> > > > > > > >   		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> > > > > > > >   							vq->event_flags_shadow);
> > > > > > > >   	}
> > > > > > > > @@ -1172,15 +1196,34 @@ static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned last_used_idx)
> > > > > > > >   static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
> > > > > > > >   {
> > > > > > > >   	struct vring_virtqueue *vq = to_vvq(_vq);
> > > > > > > > +	u16 bufs, used_idx, wrap_counter;
> > > > > > > >   	START_USE(vq);
> > > > > > > >   	/* We optimistically turn back on interrupts, then check if there was
> > > > > > > >   	 * more to do. */
> > > > > > > > +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> > > > > > > > +	 * either clear the flags bit or point the event index at the next
> > > > > > > > +	 * entry. Always update the event index to keep code simple. */
> > > > > > > > +
> > > > > > > > +	/* TODO: tune this threshold */
> > > > > > > > +	bufs = (u16)(vq->next_avail_idx - vq->last_used_idx) * 3 / 4;
> > > > > > > > +
> > > > > > > > +	used_idx = vq->last_used_idx + bufs;
> > > > > > > > +	wrap_counter = vq->wrap_counter;
> > > > > > > > +
> > > > > > > > +	if (used_idx >= vq->vring_packed.num) {
> > > > > > > > +		used_idx -= vq->vring_packed.num;
> > > > > > > > +		wrap_counter ^= 1;
> > > > > > > > +	}
> > > > > > > > +
> > > > > > > > +	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
> > > > > > > > +			used_idx | (wrap_counter << 15));
> > > > > > > >   	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> > > > > > > >   		virtio_wmb(vq->weak_barriers);
> > > > > > > > -		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> > > > > > > > +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
> > > > > > > > +						     VRING_EVENT_F_ENABLE;
> > > > > > > >   		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> > > > > > > >   							vq->event_flags_shadow);
> > > > > > > >   	}
> > > > > > > > @@ -1822,8 +1865,10 @@ void vring_transport_features(struct virtio_device *vdev)
> > > > > > > >   		switch (i) {
> > > > > > > >   		case VIRTIO_RING_F_INDIRECT_DESC:
> > > > > > > >   			break;
> > > > > > > > +#if 0
> > > > > > > >   		case VIRTIO_RING_F_EVENT_IDX:
> > > > > > > >   			break;
> > > > > > > > +#endif
> > > > > > > >   		case VIRTIO_F_VERSION_1:
> > > > > > > >   			break;
> > > > > > > >   		case VIRTIO_F_IOMMU_PLATFORM:
> > > > > > > 

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