Re: [Qemu-devel] [PATCH for-2.9 2/2] intel_iommu: extend supported guest aw to 48 bits

[Peter Xu <peterx@redhat.com>]

On Sun, Dec 11, 2016 at 05:13:45AM +0200, Michael S. Tsirkin wrote:
> On Wed, Dec 07, 2016 at 01:52:45PM +0800, Peter Xu wrote:
> > Previously vt-d codes only supports 39 bits iova address width. It won't
> > be hard to extend it to 48 bits.
> > 
> > After enabling this, we should be able to map larger iova addresses.
> > 
> > To check whether 48 bits aw is enabled, we can grep in the guest dmesg
> > with line: "dmar: Host address width 48" (previously it was 39).
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> I suspect we can't do this for old machine types.
> Need to behave in compatible ways.

Sure. I can do that.

Btw, is vt-d iommu still in experimental stage? I am just thinking
whether it'll be overkill we add lots of tunables before we have one
stable and mature vt-d emulation.

> Also, is 48 always enough? 5 level with 57 bits
> is just around the corner.

Please refer to the discussion with Jason - looks like vt-d spec
currently supports only 39/48 bits address width? Please correct if I
made a mistake.

> And is it always supported? for things like vfio
> to work, don't we need to check what does host support?

Hmm, yes, we should do that. But until now, we still don't have a
complete vfio support. IMHO we can postpone this issue until vfio is
fully supported.

Thanks,

-- peterx