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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 30, 2017 at 10:33:50AM -0800, Prasad Singamsetty wrote:
> 
> 
> On 11/29/2017 7:25 PM, Peter Xu wrote:
> > 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.
> 
> Thanks Peter, for the clarification and pointer to this. I am still
> new to this but learning on how this works or how this is used in
> use cases like Live Migration.
> 
> Are you suggesting that we change the default to 48 bits in the
> next release (2.12)?
> 
> User need to specify an older machine type  (pc-q35-2.11 or older)
> to get the old default value of 39 bits. This still requires the
> patch I proposed to support compatibility for older releases
> except the introduction of the new property (x-aw-bits).

Yes. If you see a reason for users to limit it to 39 bits,
we can make it a supported property (not starting
with x-). If it's only for live migration, we can
use a non-supported property (with x-).

> 
> > 
> > > 
> > > > 
> > > > > 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).
> > 
> 
> If we change the default value to 48 bits, I assume there is
> no need for this property and user is expected to use an
> older machine type based on the release to get the old
> default. Is this correct?
> 
> Thanks.
> --Prasad


No, users use an older machine type to get migration from old
machine types. If someone might actually want 39 bit for
some reason, we need it as a supported property.