Subject: Re: [PATCH vhost v10 07/10] virtio_ring: introduce helpers for premapped

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 5 Jun 2023 01:38:48 -0400

On Mon, Jun 05, 2023 at 10:06:51AM +0800, Xuan Zhuo wrote:
> On Sun, 4 Jun 2023 09:45:14 -0400, "Michael S. Tsirkin" <[email-masked]> wrote:
> > On Fri, Jun 02, 2023 at 05:22:03PM +0800, Xuan Zhuo wrote:
> > > This patch introduces three helpers for premapped mode.
> > >
> > > * virtqueue_get_buf_premapped
> > > * virtqueue_detach_unused_buf_premapped
> > >
> > > The above helpers work like the non-premapped funcs. But a cursor is
> > > passed.
> > >
> > > virtqueue_detach is used to get the dma info of the last buf by
> > >   cursor.
> >
> > This isn't very clear from the description but virtqueue_detach is
> > also introduced by this patch as opposed to being used.
> >
> >
> > > Signed-off-by: Xuan Zhuo <[email-masked]>
> > > ---
> > >  drivers/virtio/virtio_ring.c | 83 ++++++++++++++++++++++++++++++++++++
> > >  include/linux/virtio.h       | 10 +++++
> > >  2 files changed, 93 insertions(+)
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index cbc22daae7e1..6771b9661798 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -2555,6 +2555,66 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
> > >  	return virtqueue_get_buf_ctx(_vq, len, NULL);
> > >  }
> > >  EXPORT_SYMBOL_GPL(virtqueue_get_buf);
> > > +
> > > +/**
> > > + * virtqueue_get_buf_premapped - get the next used buffer
> > > + * @_vq: the struct virtqueue we're talking about.
> > > + * @len: the length written into the buffer
> > > + * @ctx: extra context for the token
> > > + * @cursor: detach cursor
> > > + *
> > > + * If the device wrote data into the buffer, @len will be set to the
> > > + * amount written.  This means you don't need to clear the buffer
> > > + * beforehand to ensure there's no data leakage in the case of short
> > > + * writes.
> > > + *
> > > + * Caller must ensure we don't call this with other virtqueue
> > > + * operations at the same time (except where noted).
> > > + *
> > > + * This is used for the premapped vq. The cursor is passed by the dirver, that
> > > + * is used for virtqueue_detach. That will be initialized by virtio core
> > > + * internally.
> > > + *
> > > + * Returns NULL if there are no used buffers, or the "data" token
> > > + * handed to virtqueue_add_*().
> > > + */
> > > +void *virtqueue_get_buf_premapped(struct virtqueue *_vq, unsigned int *len,
> > > +				  void **ctx,
> > > +				  struct virtqueue_detach_cursor *cursor)
> > > +{
> > > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > > +
> > > +	return vq->packed_ring ? virtqueue_get_buf_ctx_packed(_vq, len, ctx, cursor) :
> > > +				 virtqueue_get_buf_ctx_split(_vq, len, ctx, cursor);
> > > +}
> > > +EXPORT_SYMBOL_GPL(virtqueue_get_buf_premapped);
> > > +
> > > +/**
> > > + * virtqueue_detach - get the dma info of last buf
> >
> > detach what from what then?
> > I am guessing this is not the only thing this function does?
> > sounds like a bad name for a function.
> 
> Let me think of a good name
> 
> >
> > > + * @_vq: the struct virtqueue we're talking about.
> > > + * @cursor: detach cursor
> > > + * @addr: the dma address
> >
> > what address?  it's the 1st time you mention an address ...
> 
> Will fix.
> 
> 
> >
> > > + * @len: the length of the dma address
> > > + * @dir: the direction of the dma address
> > > + *
> > > + * This is used for the premapped vq. The cursor is initialized by
> > > + * virtqueue_get_buf_premapped or virtqueue_detach_unused_buf_premapped.
> > > + *
> > > + * Returns:
> > > + * -EAGAIN: there are more dma info, this function should be called more.
> >
> > here too, pls don't return -EAGAIN not in an error case.
> > something like "1" will do.
> 
> While I agree with you, -EAGAIN seems to be a commonly used method.

Where is it used like this? A typical use is e.g. in read(2):

      EAGAIN The file descriptor fd refers to a file other than a socket and has been marked nonblocking (O_NONBLOCK), and  the  read
              would block.  See open(2) for further details on the O_NONBLOCK flag.

a better analog here is read filling up all its buffer, in which
case it returns the # of bytes returned.

> How about we
> return EAGAIN instead of -EAGAIN ?
> 
> Thanks.
> 
> 
> 
> >
> > > + * -EINVAL: the process is done, should not call this function
> > > + * 0: no more dma info
> > > + */
> > > +int virtqueue_detach(struct virtqueue *_vq, struct virtqueue_detach_cursor *cursor,
> > > +		     dma_addr_t *addr, u32 *len, enum dma_data_direction *dir)
> > > +{
> > > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > > +
> > > +	return vq->packed_ring ? virtqueue_detach_packed(_vq, cursor, addr, len, dir) :
> > > +				 virtqueue_detach_split(_vq, cursor, addr, len, dir);
> > > +}
> > > +EXPORT_SYMBOL_GPL(virtqueue_detach);
> > > +
> > >  /**
> > >   * virtqueue_disable_cb - disable callbacks
> > >   * @_vq: the struct virtqueue we're talking about.
> > > @@ -2682,6 +2742,29 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
> > >  }
> > >  EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
> > >
> > > +/**
> > > + * virtqueue_detach_unused_buf_premapped - detach first unused buffer
> > > + * @_vq: the struct virtqueue we're talking about.
> > > + * @cursor: detach cursor
> > > + *
> > > + * This is used for the premapped vq. The cursor is passed by the dirver, that
> > > + * is used for virtqueue_detach. That will be initialized by virtio core
> > > + * internally.
> > > + *
> > > + * Returns NULL or the "data" token handed to virtqueue_add_*().
> > > + * This is not valid on an active queue; it is useful for device
> > > + * shutdown or the reset queue.
> > > + */
> > > +void *virtqueue_detach_unused_buf_premapped(struct virtqueue *_vq,
> > > +					    struct virtqueue_detach_cursor *cursor)
> > > +{
> > > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > > +
> > > +	return vq->packed_ring ? virtqueue_detach_unused_buf_packed(_vq, cursor) :
> > > +				 virtqueue_detach_unused_buf_split(_vq, cursor);
> > > +}
> > > +EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf_premapped);
> > > +
> > >  static inline bool more_used(const struct vring_virtqueue *vq)
> > >  {
> > >  	return vq->packed_ring ? more_used_packed(vq) : more_used_split(vq);
> > > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > > index 7f137c7a9034..0a11c5b32fe5 100644
> > > --- a/include/linux/virtio.h
> > > +++ b/include/linux/virtio.h
> > > @@ -3,6 +3,7 @@
> > >  #define _LINUX_VIRTIO_H
> > >  /* Everything a virtio driver needs to work with any particular virtio
> > >   * implementation. */
> > > +#include <linux/dma-mapping.h>
> > >  #include <linux/types.h>
> > >  #include <linux/scatterlist.h>
> > >  #include <linux/spinlock.h>
> > > @@ -88,6 +89,10 @@ void *virtqueue_get_buf(struct virtqueue *vq, unsigned int *len);
> > >  void *virtqueue_get_buf_ctx(struct virtqueue *vq, unsigned int *len,
> > >  			    void **ctx);
> > >
> > > +void *virtqueue_get_buf_premapped(struct virtqueue *_vq, unsigned int *len,
> > > +				  void **ctx,
> > > +				  struct virtqueue_detach_cursor *cursor);
> > > +
> > >  void virtqueue_disable_cb(struct virtqueue *vq);
> > >
> > >  bool virtqueue_enable_cb(struct virtqueue *vq);
> > > @@ -101,6 +106,8 @@ bool virtqueue_poll(struct virtqueue *vq, unsigned);
> > >  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
> > >
> > >  void *virtqueue_detach_unused_buf(struct virtqueue *vq);
> > > +void *virtqueue_detach_unused_buf_premapped(struct virtqueue *_vq,
> > > +					    struct virtqueue_detach_cursor *cursor);
> > >
> > >  unsigned int virtqueue_get_vring_size(const struct virtqueue *vq);
> > >
> > > @@ -114,6 +121,9 @@ dma_addr_t virtqueue_get_used_addr(const struct virtqueue *vq);
> > >  int virtqueue_resize(struct virtqueue *vq, u32 num,
> > >  		     void (*recycle)(struct virtqueue *vq, void *buf));
> > >
> > > +int virtqueue_detach(struct virtqueue *_vq, struct virtqueue_detach_cursor *cursor,
> > > +		     dma_addr_t *addr, u32 *len, enum dma_data_direction *dir);
> > > +
> > >  /**
> > >   * struct virtio_device - representation of a device using virtio
> > >   * @index: unique position on the virtio bus
> > > --
> > > 2.32.0.3.g01195cf9f
> >