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Subject: Re: [PATCH net-next v2 2/5] virtio_ring: support creating packed ring
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 <tiwei.bie@intel.com>
> >
> > 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
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