Re: [Qemu-devel] [PATCH v1 2/2] intel-iommu: Extend address width to 48 bits

[Peter Xu <peterx@redhat.com>]

On Wed, Nov 29, 2017 at 01:05:22PM -0800, Prasad Singamsetty wrote:
> Thanks Michael. Some comments below.
> 
> On 11/28/2017 9:32 AM, Michael S. Tsirkin wrote:
> > On Tue, Nov 14, 2017 at 06:13:50PM -0500, prasad.singamsetty@oracle.com wrote:
> > > From: Prasad Singamsetty <prasad.singamsetty@oracle.com>
> > > 
> > > The current implementation of Intel IOMMU code only supports 39 bits
> > > iova address width. This patch provides a new parameter (x-aw-bits)
> > > for intel-iommu to extend its address width to 48 bits but keeping the
> > > default the same (39 bits). The reason for not changing the default
> > > is to avoid potential compatibility problems
> > 
> > You can change the default, just make it 39 for existing machine types.
> 
> I think introducing a new machine type is not appropriate as this
> is an implementation limitation for the existing machine type.
> Currently q35 is the only machine type that supports intel-iommu.
> And we want to retain the current default behavior for q35 to avoid
> any new issues with live migration.

I guess "existing machine type" means e.g. pc-q35-2.11 and older ones,
rather than creating another machine type in parallel with q35.  So we
can set 48 bits as default on upcoming pc-q35-2.12 machines, while
keep the 39 bits on the old ones.

Please refer to include/hw/compat.h.

> 
> > 
> > > with live migration of
> > > intel-iommu enabled QEMU guest. The only valid values for 'x-aw-bits'
> > > parameter are 39 and 48.
> > 
> > I'd rather make it a boolean then.
> 
> Right. It seems Intel already has additional sizes supported so keeping
> it as an integer seems better.

Yes, considering that 5-level IOMMUs are coming (AFAIK).

-- 
Peter Xu