Re: kvm PCI assignment & VFIO ramblings

On Sun, Aug 28, 2011 at 04:14:00PM +0300, Avi Kivity wrote:

On 08/26/2011 12:24 PM, Roedel, Joerg wrote:

quoted

The biggest problem with this approach is that it has to happen in the
context of the given process. Linux can't really modify an mm which
which belong to another context in a safe way.

Is use_mm() insufficient?

Yes, it introduces a set of race conditions when a process that already
has an mm wants to take over another processes mm temporarily (and when
use_mm is modified to actually provide this functionality). It is only
save when used from kernel-thread context.

One example:

	Process A		Process B			Process C
	.			.				.
	.		<--	takes A->mm			.
	.			and assignes as B->mm		.
	.			.			-->	Wants to take
	.			.				B->mm, but gets
								A->mm now

This can't be secured by a lock, because it introduces potential
A->B<-->B->A lock problem when two processes try to take each others mm.
It could probably be solved by a task->real_mm pointer, havn't thought
about this yet...

	Joerg