On Wed, Feb 04, 2015 at 02:11:20AM -0800, Nicholas A. Bellinger wrote:
> On Tue, 2015-02-03 at 23:56 +0000, Al Viro wrote:
> > On Tue, Feb 03, 2015 at 06:29:59AM +0000, Nicholas A. Bellinger wrote:
> > > + * Copy over the virtio-scsi request header, which when
> > > + * ANY_LAYOUT is enabled may span multiple iovecs, or a
> > > + * single iovec may contain both the header + outgoing
> > > + * WRITE payloads.
> > > + *
> > > + * copy_from_iter() is modifying the iovecs as copies over
> > > + * req_size bytes into req, so the returned out_iter.iov[0]
> > > + * will contain the correct start + offset of the outgoing
> > > + * WRITE payload, if DMA_TO_DEVICE is set.
> >
> > It does no such thing. What it does, though, is changing out_iter so
> > that subsequent copy_from_iter() will return the data you want. Note
> > that out_iter.iov[0] will contain the correct _segment_ of that vector,
> > with the data you want at out_iter.iov_offset bytes from the beginning
> > of that segment. .iov may come to point to subsequent segments and
> > .iov_offset
> > keeps changing, but segments themselves are never changed.
>
> Yes, sorry. Updating that comment to read:
>
> /*
> * Copy over the virtio-scsi request header, which for a
> * ANY_LAYOUT enabled guest may span multiple iovecs, or a
> * single iovec may contain both the header + outgoing
> * WRITE payloads.
> *
> * copy_from_iter() copies over req_size bytes, and sets up
> * out_iter.iov[0] + out_iter.iov_offset to contain the start
> * of the outgoing WRITE payload, if DMA_TO_DEVICE is set.
> */
I'm still confused wrt what this refers to.
You don't actually play with iovs directly anymore,
why bother explaining what happens to the underlying iov?
Can we just say
copy_from_iter will advance out_iter, so that it will point
at the start of the outgoing WRITE payload, if DMA_TO_DEVICE is
set.
Seems clearer to me.
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