Re: [PATCH v2 5/5] iommu: Allow default domain type to be set on the kernel command line

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Tue, Mar 21, 2017 at 05:46:29PM +0000, Robin Murphy wrote:
> On 21/03/17 17:21, Will Deacon wrote:
> > On Tue, Mar 21, 2017 at 04:45:27PM +0100, Joerg Roedel wrote:
> >> On Fri, Mar 10, 2017 at 08:49:36PM +0000, Will Deacon wrote:
> >>> @@ -1014,8 +1027,8 @@ struct iommu_group *iommu_group_get_for_dev(struct device *dev)
> >>>  	 * IOMMU driver.
> >>>  	 */
> >>>  	if (!group->default_domain) {
> >>> -		group->default_domain = __iommu_domain_alloc(dev->bus,
> >>> -							     IOMMU_DOMAIN_DMA);
> >>> +		group->default_domain =
> >>> +			__iommu_domain_alloc(dev->bus, iommu_def_domain_type);
> >>
> >> It would be good to have a fall-back here if we are talking to an IOMMU
> >> driver that uses default domains, but does not support identity-mapped
> >> domains (yet). Exynos and Rockchip IOMMU drivers seem to fall into this
> >> category. A dev_warn() also makes sense in case allocating a identity
> >> domain fails.
> > 
> > Sure, something like the diff below?
> > 
> > Will
> > 
> > --->8
> > 
> > 
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 42a842e3f95f..f787626a745d 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -1027,10 +1027,19 @@ struct iommu_group *iommu_group_get_for_dev(struct device *dev)
> >  	 * IOMMU driver.
> >  	 */
> >  	if (!group->default_domain) {
> > -		group->default_domain =
> > -			__iommu_domain_alloc(dev->bus, iommu_def_domain_type);
> > +		struct iommu_domain *dom;
> > +
> > +		dom = __iommu_domain_alloc(dev->bus, iommu_def_domain_type);
> > +		if (!dom) {
> > +			dev_warn(dev,
> > +				 "failed to allocate default IOMMU domain of type %u; falling back to IOMMU_DOMAIN_DMA",
> > +				 iommu_def_domain_type);
> 
> Conversely, that's going to be noisy if iommu_def_domain_type was
> IOMMU_DOMAIN_DMA to begin with. I think it makes sense to warn if the
> user asked for a specific default domain type on the command line and
> that didn't work, but maybe not to bother otherwise. Plus, if they asked
> for passthrough, then not allocating a default domain at all is probably
> closer to the desired result than installing a DMA ops domain would be.

You're right -- I'll hack this about to check if the default type isn't
DOMAIN_DMA before warning about the allocation failure.

Cheers,

Will