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

[Peter Xu <peterx@redhat.com>]

On Thu, Nov 30, 2017 at 11:12:48AM -0800, Prasad Singamsetty wrote:
> 
> 
> On 11/30/2017 10:56 AM, Michael S. Tsirkin wrote:
> > 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.

You can refer to similar commit, like 048a2e8869.

But, I think I was wrong - you should better do the addition to
include/hw/i386/pc.h (see e.g. PC_COMPAT_2_10) rather than compat.h,
since Intel vIOMMU is only used for PC machines.

And... you may also need to create that PC_COMPAT_2_11 macro after
2.11 is released.  For that you can refer to a6fd5b0e050a.

Anyway, I think this "set default" work can be postponed after recent
release, which can be a separate work besides current series.

> > > 
> > > 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-).
> I think it is only for Live Migration case, we need to
> support the old default value of 39 bits.
> 
> Do you see any need to keep a non-supported property?
> It may be useful for developers.

IMHO if it's for developers x-* would be good enough.  Thanks,

-- 
Peter Xu