Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers

Il 18/12/2012 14:36, Michael S. Tsirkin ha scritto:

Some comments without arguing about whether the performance
benefit is worth it.

On Tue, Dec 18, 2012 at 01:32:48PM +0100, Paolo Bonzini wrote:

quoted

diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index cf8adb1..39d56c4 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h

@@ -7,6 +7,7 @@
 #include <linux/spinlock.h>
 #include <linux/device.h>
 #include <linux/mod_devicetable.h>
+#include <linux/dma-direction.h>
 #include <linux/gfp.h>
 
 /**

@@ -40,6 +41,26 @@ int virtqueue_add_buf(struct virtqueue *vq,
 		      void *data,
 		      gfp_t gfp);
 
+struct virtqueue_buf {
+	struct virtqueue *vq;
+	struct vring_desc *indirect, *tail;

This is wrong: virtio.h does not include virito_ring.h,
and it shouldn't by design depend on it.

quoted

+	int head;
+};
+

Can't we track state internally to the virtqueue?
Exposing it seems to buy us nothing since you can't
call add_buf between start and end anyway.

I wanted to keep the state for these functions separate from the rest.
I don't think it makes much sense to move it to struct virtqueue unless
virtqueue_add_buf is converted to use the new API (doesn't make much
sense, could even be a tad slower).

On the other hand moving it there would eliminate the dependency on
virtio_ring.h.  Rusty, what do you think?

quoted

+int virtqueue_start_buf(struct virtqueue *_vq,
+			struct virtqueue_buf *buf,
+			void *data,
+			unsigned int count,
+			unsigned int count_sg,
+			gfp_t gfp);
+
+void virtqueue_add_sg(struct virtqueue_buf *buf,
+		      struct scatterlist sgl[],
+		      unsigned int count,
+		      enum dma_data_direction dir);
+

And idea: in practice virtio scsi seems to always call sg_init_one, no?
So how about we pass in void* or something and avoid using sg and count?
This would make it useful for -net BTW.

It also passes the scatterlist from the LLD.  It calls sg_init_one for
the request/response headers.

Paolo

quoted

+void virtqueue_end_buf(struct virtqueue_buf *buf);
+
 void virtqueue_kick(struct virtqueue *vq);
 
 bool virtqueue_kick_prepare(struct virtqueue *vq);
-- 
1.7.1