RE: [PATCH 3/5] iommu: Introduce the domain op enforce_cache_coherency()

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Wednesday, April 6, 2022 6:58 AM
> 
> On Tue, Apr 05, 2022 at 01:50:36PM -0600, Alex Williamson wrote:
> > >
> > > +static bool intel_iommu_enforce_cache_coherency(struct
> iommu_domain *domain)
> > > +{
> > > +	struct dmar_domain *dmar_domain = to_dmar_domain(domain);
> > > +
> > > +	if (!dmar_domain->iommu_snooping)
> > > +		return false;
> > > +	dmar_domain->enforce_no_snoop = true;
> > > +	return true;
> > > +}
> >
> > Don't we have issues if we try to set DMA_PTE_SNP on DMARs that don't
> > support it, ie. reserved register bit set in pte faults?
> 
> The way the Intel driver is setup that is not possible. Currently it
> does:
> 
>  static bool intel_iommu_capable(enum iommu_cap cap)
>  {
> 	if (cap == IOMMU_CAP_CACHE_COHERENCY)
> 		return domain_update_iommu_snooping(NULL);
> 
> Which is a global property unrelated to any device.
> 
> Thus either all devices and all domains support iommu_snooping, or
> none do.
> 
> It is unclear because for some reason the driver recalculates this
> almost constant value on every device attach..

The reason is simply because iommu capability is a global flag 😉

when the cap is removed by this series I don't think we need keep that
global recalculation thing.

Thanks
Kevin
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