Re: [Qemu-devel] [PATCH 0/3] memory: add IOMMU notifier type

[Peter Xu]

On Tue, Sep 06, 2016 at 03:06:17PM +1000, David Gibson wrote:
> On Mon, Sep 05, 2016 at 03:21:18PM +0800, Peter Xu wrote:
> > In the thread:
> > 
> >   https://lists.gnu.org/archive/html/qemu-devel/2016-09/msg00254.html
> > 
> > Alex proposed a way for vhost DMAR to be enabled without breaking
> > existing protections on vIOMMU and device assignments. This series
> > tried to implement the idea, by introducing a IOMMU notifier type for
> > each IOMMU memory region.
> 
> Hrm, I'm pretty dubious about this concept, since it's basically just
> an interim hack for an incomplete notifier implementation on x86.
> What makes just fixing the notifier so difficult?

Aviv is working on the full notifier support for that. It's been
months since his last post though. If he cannot continue it (due to
any reason), I can take it over. But for now, we may still need to
wait for his patches to fully enable a complete notifier mechanism.

I don't know how POWER works to provide a complete notifier, and
whether POWER can selectively enable the notified items... But for
Intel VT-d, it provided two choices: by default, only cache
invalidations are notified (even, we can disalbe cache invaliations),
but if one want to have a complete notifier, just set the CM bit to 1.
So I just think it'll be cool if we can support both cases. E.g., for
vhost, it does not need to be notified with newly added entries, but
only cache invalidations. IMHO we can't just force vhost to use a
complete notifier while actually it only needs part of it.

Thanks,

-- peterx