Subject: Re: [PATCH net-next v2 2/5] virtio_ring: support creating packed ring

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 12 Sep 2018 08:45:19 -0400

On Mon, Sep 10, 2018 at 10:28:37AM +0800, Tiwei Bie wrote:
> On Fri, Sep 07, 2018 at 10:03:24AM -0400, Michael S. Tsirkin wrote:
> > On Wed, Jul 11, 2018 at 10:27:08AM +0800, Tiwei Bie wrote:
> > > This commit introduces the support for creating packed ring.
> > > All split ring specific functions are added _split suffix.
> > > Some necessary stubs for packed ring are also added.
> > > 
> > > Signed-off-by: Tiwei Bie <[email-masked]>
> > 
> > I'd rather have a patch just renaming split functions, then
> > add all packed stuff in as a separate patch on top.
> 
> Sure, I will do that.
> 
> > 
> > 
> > > ---
> > >  drivers/virtio/virtio_ring.c | 801 +++++++++++++++++++++++------------
> > >  include/linux/virtio_ring.h  |   8 +-
> > >  2 files changed, 546 insertions(+), 263 deletions(-)
> > > 
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index 814b395007b2..c4f8abc7445a 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -60,11 +60,15 @@ struct vring_desc_state {
> > >  	struct vring_desc *indir_desc;	/* Indirect descriptor, if any. */
> > >  };
> > >  
> > > +struct vring_desc_state_packed {
> > > +	int next;			/* The next desc state. */
> > 
> > So this can go away with IN_ORDER?
> 
> Yes. If IN_ORDER is negotiated, next won't be needed anymore.
> Currently, IN_ORDER isn't included in this patch set, because
> some changes for split ring are needed to make sure that it
> will use the descs in the expected order. After that,
> optimizations can be done for both of split ring and packed
> ring respectively.
> 
> > 
> > > +};
> > > +
> > >  struct vring_virtqueue {
> > >  	struct virtqueue vq;
> > >  
> > > -	/* Actual memory layout for this queue */
> > > -	struct vring vring;
> > > +	/* Is this a packed ring? */
> > > +	bool packed;
> > >  
> > >  	/* Can we use weak barriers? */
> > >  	bool weak_barriers;
> > > @@ -86,11 +90,39 @@ struct vring_virtqueue {
> > >  	/* Last used index we've seen. */
> > >  	u16 last_used_idx;
> > >  
> > > -	/* Last written value to avail->flags */
> > > -	u16 avail_flags_shadow;
> > > +	union {
> > > +		/* Available for split ring */
> > > +		struct {
> > > +			/* Actual memory layout for this queue. */
> > > +			struct vring vring;
> > >  
> > > -	/* Last written value to avail->idx in guest byte order */
> > > -	u16 avail_idx_shadow;
> > > +			/* Last written value to avail->flags */
> > > +			u16 avail_flags_shadow;
> > > +
> > > +			/* Last written value to avail->idx in
> > > +			 * guest byte order. */
> > > +			u16 avail_idx_shadow;
> > > +		};
> > 
> > Name this field split so it's easier to detect misuse of e.g.
> > packed fields in split code?
> 
> Good point, I'll do that.
> 
> > 
> > > +
> > > +		/* Available for packed ring */
> > > +		struct {
> > > +			/* Actual memory layout for this queue. */
> > > +			struct vring_packed vring_packed;
> > > +
> > > +			/* Driver ring wrap counter. */
> > > +			bool avail_wrap_counter;
> > > +
> > > +			/* Device ring wrap counter. */
> > > +			bool used_wrap_counter;
> > > +
> > > +			/* Index of the next avail descriptor. */
> > > +			u16 next_avail_idx;
> > > +
> > > +			/* Last written value to driver->flags in
> > > +			 * guest byte order. */
> > > +			u16 event_flags_shadow;
> > > +		};
> > > +	};
> [...]
> > > +
> > > +/*
> > > + * The layout for the packed ring is a continuous chunk of memory
> > > + * which looks like this.
> > > + *
> > > + * struct vring_packed {
> > > + *	// The actual descriptors (16 bytes each)
> > > + *	struct vring_packed_desc desc[num];
> > > + *
> > > + *	// Padding to the next align boundary.
> > > + *	char pad[];
> > > + *
> > > + *	// Driver Event Suppression
> > > + *	struct vring_packed_desc_event driver;
> > > + *
> > > + *	// Device Event Suppression
> > > + *	struct vring_packed_desc_event device;
> > > + * };
> > > + */
> > 
> > Why not just allocate event structures separately?
> > Is it a win to have them share a cache line for some reason?
> 
> Will do that.
> 
> > 
> > > +static inline void vring_init_packed(struct vring_packed *vr, unsigned int num,
> > > +				     void *p, unsigned long align)
> > > +{
> > > +	vr->num = num;
> > > +	vr->desc = p;
> > > +	vr->driver = (void *)ALIGN(((uintptr_t)p +
> > > +		sizeof(struct vring_packed_desc) * num), align);
> > > +	vr->device = vr->driver + 1;
> > > +}
> > 
> > What's all this about alignment? Where does it come from?
> 
> It comes from the `vring_align` parameter of vring_create_virtqueue()
> and vring_new_virtqueue():
> 
> https://github.com/torvalds/linux/blob/a49a9dcce802/drivers/virtio/virtio_ring.c#L1061
> https://github.com/torvalds/linux/blob/a49a9dcce802/drivers/virtio/virtio_ring.c#L1123

Note the TODO - we just never got to fixing it. It would be great to fix
this for virtio 1 rings, but if not - at least let's not add this
stuff for packed rings.

> Should I just ignore it in packed ring?
> 
> CCW defined this:
> 
> #define KVM_VIRTIO_CCW_RING_ALIGN 4096
> 
> I'm not familiar with CCW. Currently, in this patch set, packed ring
> isn't enabled on CCW dues to some legacy accessors are not implemented
> in packed ring yet.

Then you need to take steps not to negotiate this feature bit for
ccw drivers.

> > 
> > > +
> > > +static inline unsigned vring_size_packed(unsigned int num, unsigned long align)
> > > +{
> > > +	return ((sizeof(struct vring_packed_desc) * num + align - 1)
> > > +		& ~(align - 1)) + sizeof(struct vring_packed_desc_event) * 2;
> > > +}
> [...]
> > > @@ -1129,10 +1388,17 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
> > >  				      void (*callback)(struct virtqueue *vq),
> > >  				      const char *name)
> > >  {
> > > -	struct vring vring;
> > > -	vring_init(&vring, num, pages, vring_align);
> > > -	return __vring_new_virtqueue(index, vring, vdev, weak_barriers, context,
> > > -				     notify, callback, name);
> > > +	union vring_union vring;
> > > +	bool packed;
> > > +
> > > +	packed = virtio_has_feature(vdev, VIRTIO_F_RING_PACKED);
> > > +	if (packed)
> > > +		vring_init_packed(&vring.vring_packed, num, pages, vring_align);
> > > +	else
> > > +		vring_init(&vring.vring_split, num, pages, vring_align);
> > 
> > 
> > vring_init in the UAPI header is more or less a bug.
> > I'd just stop using it, keep it around for legacy userspace.
> 
> Got it. I'd like to do that. Thanks.
> 
> > 
> > > +
> > > +	return __vring_new_virtqueue(index, vring, packed, vdev, weak_barriers,
> > > +				     context, notify, callback, name);
> > >  }
> > >  EXPORT_SYMBOL_GPL(vring_new_virtqueue);
> > >  
> > > @@ -1142,7 +1408,9 @@ void vring_del_virtqueue(struct virtqueue *_vq)
> > >  
> > >  	if (vq->we_own_ring) {
> > >  		vring_free_queue(vq->vq.vdev, vq->queue_size_in_bytes,
> > > -				 vq->vring.desc, vq->queue_dma_addr);
> > > +				 vq->packed ? (void *)vq->vring_packed.desc :
> > > +					      (void *)vq->vring.desc,
> > > +				 vq->queue_dma_addr);
> > >  	}
> > >  	list_del(&_vq->list);
> > >  	kfree(vq);
> > > @@ -1184,7 +1452,7 @@ unsigned int virtqueue_get_vring_size(struct virtqueue *_vq)
> > >  
> > >  	struct vring_virtqueue *vq = to_vvq(_vq);
> > >  
> > > -	return vq->vring.num;
> > > +	return vq->packed ? vq->vring_packed.num : vq->vring.num;
> > >  }
> > >  EXPORT_SYMBOL_GPL(virtqueue_get_vring_size);
> > >  
> > > @@ -1227,6 +1495,10 @@ dma_addr_t virtqueue_get_avail_addr(struct virtqueue *_vq)
> > >  
> > >  	BUG_ON(!vq->we_own_ring);
> > >  
> > > +	if (vq->packed)
> > > +		return vq->queue_dma_addr + ((char *)vq->vring_packed.driver -
> > > +				(char *)vq->vring_packed.desc);
> > > +
> > >  	return vq->queue_dma_addr +
> > >  		((char *)vq->vring.avail - (char *)vq->vring.desc);
> > >  }
> > > @@ -1238,11 +1510,16 @@ dma_addr_t virtqueue_get_used_addr(struct virtqueue *_vq)
> > >  
> > >  	BUG_ON(!vq->we_own_ring);
> > >  
> > > +	if (vq->packed)
> > > +		return vq->queue_dma_addr + ((char *)vq->vring_packed.device -
> > > +				(char *)vq->vring_packed.desc);
> > > +
> > >  	return vq->queue_dma_addr +
> > >  		((char *)vq->vring.used - (char *)vq->vring.desc);
> > >  }
> > >  EXPORT_SYMBOL_GPL(virtqueue_get_used_addr);
> > >  
> > > +/* Only available for split ring */
> > >  const struct vring *virtqueue_get_vring(struct virtqueue *vq)
> > >  {
> > >  	return &to_vvq(vq)->vring;
> > > diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> > > index fab02133a919..992142b35f55 100644
> > > --- a/include/linux/virtio_ring.h
> > > +++ b/include/linux/virtio_ring.h
> > > @@ -60,6 +60,11 @@ static inline void virtio_store_mb(bool weak_barriers,
> > >  struct virtio_device;
> > >  struct virtqueue;
> > >  
> > > +union vring_union {
> > > +	struct vring vring_split;
> > > +	struct vring_packed vring_packed;
> > > +};
> > > +
> > >  /*
> > >   * Creates a virtqueue and allocates the descriptor ring.  If
> > >   * may_reduce_num is set, then this may allocate a smaller ring than
> > > @@ -79,7 +84,8 @@ struct virtqueue *vring_create_virtqueue(unsigned int index,
> > >  
> > >  /* Creates a virtqueue with a custom layout. */
> > >  struct virtqueue *__vring_new_virtqueue(unsigned int index,
> > > -					struct vring vring,
> > > +					union vring_union vring,
> > > +					bool packed,
> > >  					struct virtio_device *vdev,
> > >  					bool weak_barriers,
> > >  					bool ctx,
> > > -- 
> > > 2.18.0