Re: [Xen-devel] [PATCH] xen-mapcache: don''t unmap locked entry during mapcache invalidation

Tim Deegan
2012-03-15T17:30:07Z

At 17:14 +0000 on 15 Mar (1331831693), Stefano Stabellini
wrote:
> On Thu, 15 Mar 2012, Julien Grall wrote:
> > When an IOREQ_TYPE_INVALIDATE is sent to QEMU, it invalidates all
entry
> > of the map cache even if it''s locked.
> >
> > QEMU is not able to know that entry was invalidated, so when an IO
> > access is requested a segfault occured.
>
> The problem here is the long term mappings in QEMU that cannot easily be
> re-created.
> I am not sure whether this can cause any trouble to things like
> xenpaging.

It causes some trouble to ballooning - a guest might try to return memory
to Xen only to find that Qemu won''t let go of it.

If (as I hope is the case) qemu never has a locked mapping to something
that the guets ought to be ballooning, that''s OK.  If this happens just
because the page was recently a DMA target, then it''s not.

Cheers,

Tim.