Re: [RFC PATCH 1/3] iommu: introduce IOMMU_DOMAIN_HYP domain type for hypervisor allocation

[Alex Williamson <alex.williamson-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Wed, 2014-07-02 at 14:57 +0100, Will Deacon wrote:
> On Wed, Jul 02, 2014 at 11:49:02AM +0100, Will Deacon wrote:
> > On Tue, Jul 01, 2014 at 08:28:11PM +0100, Alex Williamson wrote:
> > > If we go the route of defining it at allocation time, I'd probably think
> > > about adding a new interface, like:
> > > 
> > > iommu_domain_alloc_with_features(struct bus_type *bus, u32 features)
> > > 
> > > Where features is a bitmap
> > > 
> > > #define IOMMU_DOMAIN_FEATURE_NESTING (1U << 0)
> > > 
> > > iommu_domain_alloc(struct bus_type *bus) would then just be a wrapper
> > > with features = 0.  If an IOMMU driver doesn't support a requested
> > > feature, it should fail so we don't have cases like KVM requesting a
> > > "HYP" domain when it doesn't need it and the IOMMU drivers don't support
> > > it.
> > > 
> > > It would be a lot less intrusive if we could use iommu_domain_set_attr()
> > > to enable nesting after allocation though.  This could return error if
> > > called after the point at which it can be easily enabled.
> > 
> > I'll take another look at set_attr, since I agree that it would be cleaner.
> 
> Ok, we quickly end up with a bootstrap problem here:
> 
>   - We need to know whether the domain is stage-1 or stage-2 before we've
>     attached any devices to it (since we can have DMA traffic once a
>     device is attached).
> 
>   - Until a device is attached, we don't know the specific SMMU instance
>     backing the domain.
> 
>   - Until we know the SMMU, we don't know whether or not we can do
>     nesting, and therefore can't handle set_attr calls.
> 
> So the two places we can provide this information are either at domain_alloc
> time or at device_attach time. I think the former makes more sense, so I'll
> look at adding a new alloc function as you suggest above.

I don't really see how using set_attr to set a domain parameter is any
different than specifying it at domain_alloc.  "Create a domain, set it
to require nesting, add devices" versus "Create a domain and set it to
require nesting, add devices".  Thanks,

Alex