Re: [Qemu-devel] [PATCH 07/13] usb: Convert usb_packet_{map, unmap} to universal DMA helpers

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2012-06-20 at 13:14 +1000, David Gibson wrote:
> So, in fact the original comment is a bit out of date.  With the
> current version of this series, then a guest attempt to invalidate
> will be delayed until the unmap occurs. 

No, this code was dropped, including the tracking of the maps, following
comments from Anthony and others. The API for providing a cancel
callback is still there but nothing will call it unless the backend does
its own tracking and decides to do so.

As it is, the race exist but:

 - It will only hurt the guest

 - And only for a very buggy guest

So the worst case is that it hurts something like kdump.


I plan to re-introduce some of the mechanisms for cancellation
eventually, but we agreed that it wasn't going to be a show stopper and
that we could work on getting that sorted in a second phase. I'm looking
at a more efficient way to deal with the tracking of the maps as well
since some devices uses them often.

Cheers,
Ben.