RE: [PATCH 3/4] iommu: Add support to change default domain of an iommu_group

["Prakhya, Sai Praneeth" <sai.praneeth.prakhya@intel.com>]

Hi Joerg,

Thanks a lot! for the review. I highly appreciate for sparing your time to review the patch :)

> On Tue, Aug 20, 2019 at 07:42:25PM -0700, Sai Praneeth Prakhya wrote:
> > +	/*
> > +	 * iommu_domain_alloc() takes "struct bus_type" as an argument which
> is
> > +	 * a member in "struct device". Changing a group's default domain type
> > +	 * deals at iommu_group level rather than device level and hence there
> > +	 * is no straight forward way to get "bus_type" of an iommu_group that
> > +	 * could be passed to iommu_domain_alloc(). So, instead of directly
> > +	 * calling iommu_domain_alloc(), use iommu_ops from previous default
> > +	 * domain.
> > +	 */
> > +	if (!prev_domain || !prev_domain->ops ||
> > +	    !prev_domain->ops->domain_alloc || !type)
> > +		return -EINVAL;
> 
> Hmm, this isn't really nice and clean, but I understand why you need it.

I agree.. It didn't look good for me either :(
But, I didn't find any other better solution. 

> I will think about a better way to get iommu_ops here.

Sure! That will be great!

> > +free_prev_domain:
> > +	/*
> > +	 * Free the existing default domain and replace it with the newly
> > +	 * created default domain. No need to set group->domain because
> > +	 * __iommu_attach_group() already does it on success.
> > +	 */
> > +	iommu_domain_free(prev_domain);
> > +	group->default_domain = new_domain;
> > +	return 0;
> 
> It isn't obvious to me from this patch, how to are the dma_ops updated when
> the default domain changes between identity and dma?

Thanks for raising this.
For intel_iommu, dma_map_ops is defined at drivers/iommu/intel-iommu.c and
all the callbacks like alloc(), map_sg() and map_page(), check if the device needs DMA mapping or not 
by calling iommu_need_mapping(). The callbacks inherently do the right thing based on the outcome.
So, essentially the dma_ops are same for dma/identity domain.

I just realized (sorry!) that other iommu drivers (Eg: AMD) doesn't do it the same way i.e. looks like the callbacks 
aren't checking if the device needs a dma mapping or identity mapping.
I will take a look at other iommu drivers and will handle this in V2.

Please let me know if I missed something.

> > +	/* Check if any device in the group still has a driver binded to it */
> > +	if (iommu_group_for_each_dev(group, NULL, is_driver_binded)) {
> > +		pr_err("Active drivers exist for devices in the group\n");
> > +		return -EBUSY;
> > +	}
> 
> This is racy with device driver probing code. Unfortunatly there is no clean way
> out of that either, locking all devices in the group and then do the re-attach will
> introduce a lock-inversion with group->mutex. But please put a comment here
> saying that this might race with device driver probing.

Sure! Makes sense. Will add it in V2.

Regards,
Sai
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