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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
