Re: [PATCH v4 08/17] iommufd: Always setup MSI and anforce cc on kernel-managed domains

[Nicolin Chen]

On Tue, Oct 17, 2023 at 12:53:01PM -0300, Jason Gunthorpe wrote:
> On Tue, Oct 17, 2023 at 08:52:49AM +0000, Tian, Kevin wrote:
> > I prefer to removing enforce_cc in attach fn completely then no parent
> > trick in this patch. Just keep it in hwpt_alloc and leave to iommu driver to
> > figure out the attaching compatibility:
> 
> You are basically saying to set the cc mode during creation because we
> know the idev at that moment and can tell if it should be on/off?
> 
> It seems reasonable, but I can't remember why it is in the attach path
> at the moment.

This was the commit adding it in the alloc path:
https://lore.kernel.org/linux-iommu/8e897628-61fa-b3fb-b609-44eeda11b45e@xxxxxxx/

The older code was doing a hwpt "upgrade" from !cc to cc:
-       /*
-        * Try to upgrade the domain we have, it is an iommu driver bug to
-        * report IOMMU_CAP_ENFORCE_CACHE_COHERENCY but fail
-        * enforce_cache_coherency when there are no devices attached to the
-        * domain.
-        */
-       if (idev->enforce_cache_coherency && !hwpt->enforce_cache_coherency) {
-               if (hwpt->domain->ops->enforce_cache_coherency)
-                       hwpt->enforce_cache_coherency =
-                               hwpt->domain->ops->enforce_cache_coherency(
-                                       hwpt->domain);

If we remove the enforce_cc call in the attach path and let the
driver decide whether to enforce or reject in attach_dev calls,
there seems to be no point in tracking an enforce_cache_coherency
flag in the IOMMUFD pathway but only for the VFIO_DMA_CC_IOMMU
extension check in the vfio-compat pathway?

Thanks
Nic