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

[Tim Deegan]

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.