Subject: Re: [PATCH net-next v2 1/5] virtio: add packed ring definitions

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 12 Sep 2018 08:53:20 -0400

On Mon, Sep 10, 2018 at 10:13:22AM +0800, Tiwei Bie wrote:
> On Fri, Sep 07, 2018 at 09:51:23AM -0400, Michael S. Tsirkin wrote:
> > On Wed, Jul 11, 2018 at 10:27:07AM +0800, Tiwei Bie wrote:
> > > Signed-off-by: Tiwei Bie <[email-masked]>
> > > ---
> > >  include/uapi/linux/virtio_config.h |  3 +++
> > >  include/uapi/linux/virtio_ring.h   | 43 ++++++++++++++++++++++++++++++
> > >  2 files changed, 46 insertions(+)
> > > 
> > > diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h
> > > index 449132c76b1c..1196e1c1d4f6 100644
> > > --- a/include/uapi/linux/virtio_config.h
> > > +++ b/include/uapi/linux/virtio_config.h
> > > @@ -75,6 +75,9 @@
> > >   */
> > >  #define VIRTIO_F_IOMMU_PLATFORM		33
> > >  
> > > +/* This feature indicates support for the packed virtqueue layout. */
> > > +#define VIRTIO_F_RING_PACKED		34
> > > +
> > >  /*
> > >   * Does the device support Single Root I/O Virtualization?
> > >   */
> > > diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
> > > index 6d5d5faa989b..0254a2ba29cf 100644
> > > --- a/include/uapi/linux/virtio_ring.h
> > > +++ b/include/uapi/linux/virtio_ring.h
> > > @@ -44,6 +44,10 @@
> > >  /* This means the buffer contains a list of buffer descriptors. */
> > >  #define VRING_DESC_F_INDIRECT	4
> > >  
> > > +/* Mark a descriptor as available or used. */
> > > +#define VRING_DESC_F_AVAIL	(1ul << 7)
> > > +#define VRING_DESC_F_USED	(1ul << 15)
> > > +
> > >  /* The Host uses this in used->flags to advise the Guest: don't kick me when
> > >   * you add a buffer.  It's unreliable, so it's simply an optimization.  Guest
> > >   * will still kick if it's out of buffers. */
> > > @@ -53,6 +57,17 @@
> > >   * optimization.  */
> > >  #define VRING_AVAIL_F_NO_INTERRUPT	1
> > >  
> > > +/* Enable events. */
> > > +#define VRING_EVENT_F_ENABLE	0x0
> > > +/* Disable events. */
> > > +#define VRING_EVENT_F_DISABLE	0x1
> > > +/*
> > > + * Enable events for a specific descriptor
> > > + * (as specified by Descriptor Ring Change Event Offset/Wrap Counter).
> > > + * Only valid if VIRTIO_RING_F_EVENT_IDX has been negotiated.
> > > + */
> > > +#define VRING_EVENT_F_DESC	0x2
> > > +
> > >  /* We support indirect buffer descriptors */
> > >  #define VIRTIO_RING_F_INDIRECT_DESC	28
> > >  
> > 
> > These are for the packed ring, right? Pls prefix accordingly.
> 
> How about something like this:
> 
> #define VRING_PACKED_DESC_F_AVAIL	(1u << 7)
> #define VRING_PACKED_DESC_F_USED	(1u << 15)

_F_ should be a bit number, not a shifted value.
Yes I know current virtio_ring.h is inconsistent in
that respect. changes welcome though we need to
maintain the old names for the sake of old apps.

> 
> #define VRING_PACKED_EVENT_F_ENABLE	0x0
> #define VRING_PACKED_EVENT_F_DISABLE	0x1
> #define VRING_PACKED_EVENT_F_DESC	0x2

These are values, I think they should not have _F_.
Yes I know current virtio_ring.h is inconsistent in
that respect.

> 
> > Also, you likely need macros for the wrap counters.
> 
> How about something like this:
> 
> #define VRING_PACKED_EVENT_WRAP_COUNTER_SHIFT	15

Given it's a single bit, maybe even
VRING_PACKED_EVENT_F_WRAP_CTR

> #define VRING_PACKED_EVENT_WRAP_COUNTER_MASK	\
> 			(1u << VRING_PACKED_WRAP_COUNTER_SHIFT)
> #define VRING_PACKED_EVENT_OFFSET_MASK	\
> 			((1u << VRING_PACKED_WRAP_COUNTER_SHIFT) - 1)

I'm not sure the mask is justified.

> > 
> > > @@ -171,4 +186,32 @@ static inline int vring_need_event(__u16 event_idx, __u16 new_idx, __u16 old)
> > >  	return (__u16)(new_idx - event_idx - 1) < (__u16)(new_idx - old);
> > >  }
> > >  
> > > +struct vring_packed_desc_event {
> > > +	/* Descriptor Ring Change Event Offset/Wrap Counter. */
> > > +	__virtio16 off_wrap;
> > > +	/* Descriptor Ring Change Event Flags. */
> > > +	__virtio16 flags;
> > > +};
> > > +
> > > +struct vring_packed_desc {
> > > +	/* Buffer Address. */
> > > +	__virtio64 addr;
> > > +	/* Buffer Length. */
> > > +	__virtio32 len;
> > > +	/* Buffer ID. */
> > > +	__virtio16 id;
> > > +	/* The flags depending on descriptor type. */
> > > +	__virtio16 flags;
> > > +};
> > 
> > Don't use __virtioXX types, just __leXX ones.
> 
> Got it, will do that.
> 
> > 
> > > +
> > > +struct vring_packed {
> > > +	unsigned int num;
> > > +
> > > +	struct vring_packed_desc *desc;
> > > +
> > > +	struct vring_packed_desc_event *driver;
> > > +
> > > +	struct vring_packed_desc_event *device;
> > > +};
> > > +
> > >  #endif /* _UAPI_LINUX_VIRTIO_RING_H */
> > > -- 
> > > 2.18.0