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

[Nicolin Chen]

On Tue, Sep 26, 2023 at 01:16:35AM -0700, Tian, Kevin wrote:
> > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > Sent: Thursday, September 21, 2023 3:51 PM
> >
> > From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> >
> > Now enforce_cache_coherency and msi_cookie are kernel-managed hwpt
> > things.
> > So, they should be only setup on kernel-managed domains. If the attaching
> > domain is a user-managed domain, redirect the hwpt to hwpt->parent to do
> > it correctly.
> >
> 
> No redirection. The parent should already have the configuration done
> when it's created. It shouldn't be triggered in the nesting path.

iommufd_hw_pagetable_enforce_cc() is not only called in alloc(),
but also in hwpt_attach/replace() if cc is not enforced by the
alloc() because the idev that initiates the hwpt_alloc() might
not have idev->enforce_cache_coherency. Only when another idev
that has idev->enforce_cache_coherency attaches to the shared
hwpt, the cc configuration would be done.

In a nesting case encountering the same situation above, the S2
hwpt is allocated without the iommufd_hw_pagetable_enforce_cc().
But the 2nd idev that has idev->enforce_cache_coherency might
attach directly to the S1 domain/hwpt without touching the S2
domain (for the same VM, S2 domain can be shared). In this case,
without a redirection, the iommufd_hw_pagetable_enforce_cc()
would be missed.

Any thought?

Nic