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

[Prasad Singamsetty]

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, address@hidden wrote:
> > > > From: Prasad Singamsetty <address@hidden>
> > > >
> > > > 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).


> > > > 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