Subject: Re: [RFC v5 2/5] virtio_ring: support creating packed ring

From: Tiwei Bie <[email-masked]>
Date: Tue, 29 May 2018 13:24:18 +0800

On Tue, May 29, 2018 at 10:49:11AM +0800, Jason Wang wrote:
> On 2018年05月22日 16:16, 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]>
> > ---
> >   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 71458f493cf8..f5ef5f42a7cf 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -61,11 +61,15 @@ struct vring_desc_state {
> >   	struct vring_desc *indir_desc;	/* Indirect descriptor, if any. */
> >   };
> > +struct vring_desc_state_packed {
> > +	int next;			/* The next desc state. */
> > +};
> > +
> >   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;
> > @@ -87,11 +91,39 @@ 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;
> > +		};
> > +
> > +		/* Available for packed ring */
> > +		struct {
> > +			/* Actual memory layout for this queue. */
> > +			struct vring_packed vring_packed;
> > +
> > +			/* Driver ring wrap counter. */
> > +			u8 avail_wrap_counter;
> > +
> > +			/* Device ring wrap counter. */
> > +			u8 used_wrap_counter;
> 
> How about just use boolean?

I can change it to bool if you want.

[...]
> > -static int vring_mapping_error(const struct vring_virtqueue *vq,
> > -			       dma_addr_t addr)
> > -{
> > -	if (!vring_use_dma_api(vq->vq.vdev))
> > -		return 0;
> > -
> > -	return dma_mapping_error(vring_dma_dev(vq), addr);
> > -}
> 
> It looks to me if you keep vring_mapping_error behind
> vring_unmap_one_split(), lots of changes were unncessary.
> 
[...]
> > +	}
> > +	/* That should have freed everything. */
> > +	BUG_ON(vq->vq.num_free != vq->vring.num);
> > +
> > +	END_USE(vq);
> > +	return NULL;
> > +}
> 
> I think the those copy-and-paste hunks could be avoided and the diff should
> only contains renaming of the function. If yes, it would be very welcomed
> since it requires to compare the changes verbatim otherwise.

Michael suggested to lay out the code as:

XXX_split

XXX_packed

XXX wrappers

https://lkml.org/lkml/2018/4/13/410

That's why I moved some code.

> 
> > +
> > +/*
> > + * 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;
> > + * };
> > + */
> > +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 *)(((uintptr_t)p + sizeof(struct vring_packed_desc)
> > +		* num + align - 1) & ~(align - 1));
> 
> If we choose not to go uapi, maybe we can use ALIGN() macro here?

Okay.

> 
> > +	vr->device = vr->driver + 1;
> > +}
[...]
> > +/* Only available for split ring */
> >   const struct vring *virtqueue_get_vring(struct virtqueue *vq)
> >   {
> 
> A possible issue with this is:
> 
> After commit d4674240f31f8c4289abba07d64291c6ddce51bc ("KVM: s390:
> virtio-ccw revision 1 SET_VQ"). CCW tries to use
> virtqueue_get_avail()/virtqueue_get_used(). Looks like a bug either here or
> ccw code.

Do we still need to support:

include/linux/virtio.h
/*
 * Legacy accessors -- in almost all cases, these are the wrong functions
 * to use.
 */
static inline void *virtqueue_get_desc(struct virtqueue *vq)
{
        return virtqueue_get_vring(vq)->desc;
}
static inline void *virtqueue_get_avail(struct virtqueue *vq)
{
        return virtqueue_get_vring(vq)->avail;
}
static inline void *virtqueue_get_used(struct virtqueue *vq)
{
        return virtqueue_get_vring(vq)->used;
}

in packed ring?

If so, I think maybe it's better to expose them as
symbols and implement them in virtio_ring.c.

Best regards,
Tiwei Bie