Re: [PATCH 05/10] vfio: Use IOMMU_CAP_ENFORCE_CACHE_COHERENCY for vfio_file_enforced_coherent()

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Nov 01, 2022 at 07:52:23AM +0000, Tian, Kevin wrote:
> > IOMMU_CAP_ENFORCE_CACHE_COHERENCY we know what the
> > iommu_domain will be
> > capable of without having to create it. Use this to compute
> 
> it's worth noting that the prerequisite is that vfio always enforces
> cache coherency on a domain according to the iommu capability
> of the devices attached to that domain. There is no mix of attaching
> a device supporting the cap to a domain which doesn't enforce
> coherency. With that we know what the domain will be w/o having
> to create it.

OK, I added this:

VFIO always tries to upgrade domains to enforce cache coherency, it never
attaches a device that supports enforce cache coherency to a less capable
domain, so the cap test is a sufficient proxy for the ultimate
outcome. iommufd also ensures that devices that set the cap will be
connected to enforcing domains.

> > +	/*
> > +	 * If the device does not have
> > IOMMU_CAP_ENFORCE_CACHE_COHERENCY then
> > +	 * any domain later attached to it will also not support it.
> > +	 */
> 
> also add the other part i.e. if the device does have the cap then any domain
> later attached to it will have the cap enabled. Only with both clarified
> we can safely use the device cap here.

And this:

	/*
	 * If the device does not have IOMMU_CAP_ENFORCE_CACHE_COHERENCY then
	 * any domain later attached to it will also not support it. If the cap
	 * is set then the iommu_domain eventually attached to the device/group
	 * must must use a domain with enforce_cache_coherency().
	 */

Jason