PERFORCE change 128745 for review

[Robert Watson]

http://perforce.freebsd.org/chv.cgi?CH=128745

Change 128745 by rwatson at rwatson_fledge on 2007/11/06 16:42:16

	Update two comments.

Affected files ...

.. //depot/projects/zcopybpf/src/sys/net/bpf_zerocopy.c#21 edit

Differences ...

==== //depot/projects/zcopybpf/src/sys/net/bpf_zerocopy.c#21 (text+ko) ====

@@ -74,7 +74,8 @@
  * sf_bufs.  Even though the memory is contiguous in user space, it may not
  * be mapped contiguously in the kernel (i.e., a set of physically
  * non-contiguous pages in the direct map region) so we must implement
- * scatter-gather copying.
+ * scatter-gather copying.  One significant mitigating factor is that on
+ * systems with a direct memory map, we can avoid TLB misses.
  *
  * At the front of the shared memor region is a bpf_zbuf_header, which
  * contains shared control data to allow user space and the kernel to
@@ -402,9 +403,8 @@
 }
 
 /*
- * For now, allow bpfread() to rotate the buffers, but don't perform a copy
- * operation or return a value.  If we want to copy, we'll need to implement
- * scatter-gather copying with a series of uiomove calls here.
+ * read() is unimplemented for zero-copy BPF buffers, even though in
+ * principle you could combine the two semantics.
  */
 int
 bpf_zerocopy_uiomove(struct bpf_d *d, caddr_t buf, u_int len,