Re: [PATCH 4/4] iommu/vt-d: Remove lazy allocation of domains

[Lu Baolu]

Hi,

On 3/9/19 7:49 PM, James Sewart wrote:
> Hey Lu,
>
> > On 9 Mar 2019, at 01:53, Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> wrote:
> >
> > Hi James,
> >
> > On 3/9/19 12:57 AM, James Sewart wrote:
> > > Hey Lu,
> > > > On 8 Mar 2019, at 03:09, Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>  wrote:
> > > > > > Do you mind if I work on top of your patches for further cleanups and
> > > > > > sign off a v2 together with you?
> > > > > Sure, sounds good. Iâll fixup patch 3 and have a go at integrating
> > > > > iommu_prepare_isa into get_resv_regions. This should make the initial
> > > > > domain logic here quite concise.
> > > > Here attached three extra patches which I think should be added before
> > > > PATCH 3/4, and some further cleanup changes which you can merge with
> > > > PATCH 4/4.
> > > >
> > > > ----------------
> > > >
> > > > 0006-iommu-Add-ops-entry-for-vendor-specific-default-doma.patch
> > > > 0007-iommu-vt-d-Add-is_identity_map-ops-entry.patch
> > > >
> > > > These two patches aim to add a generic method for vendor specific iommu
> > > > drivers to specify the type of the default domain for a device. Intel
> > > > iommu driver will register an ops for this since it already has its own
> > > > identity map adjudicator for a long time.
> > > This seems like a good idea, but as domain alloc is only called for the
> > > default domain on first device attached to a group, we may miss checking
> > > whether a device added later should have an identity domain. Should there
> > > be paths to downgrade a groups domain if one of the devices requires one?
> >
> > Good catch!
> >
> > This is supposed to be handled in iommu_no_mapping(). But, obviously
> > current code sticks to lazy domain allocation. I'm not sure whether
> > there are any real such cases, but we should handle it in a clean way.
> > My idea is that we could downgrade to multiple domains per group (just
> > like what we have now) in this case and print a kernel message for this.
>
> I think if a device requires an identity domain, then it should ignore
> attempts to attach to something else. A print to warn a user about this
> would be a good idea.
>
> I figure during attach: if iommu_no_mapping() then attach to si_domain and
> print, else continue with the given domain.

Yes, agree.

Best regards,
Lu Baolu