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Jason Wang

2018-May-02 02:51 UTC

[RFC v3 4/5] virtio_ring: add event idx support in packed ring

On 2018?04?25? 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 at intel.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.

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:

Tiwei Bie

2018-May-02 07:28 UTC

head link

[RFC v3 4/5] virtio_ring: add event idx support in packed ring

On Wed, May 02, 2018 at 10:51:06AM +0800, Jason Wang
wrote:> On 2018?04?25? 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 at intel.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);

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:
>

Michael S. Tsirkin

2018-May-02 13:51 UTC

head link

[RFC v3 4/5] virtio_ring: add event idx support in packed ring

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?04?25? 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 at intel.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.

> 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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