linux-kernel - Re: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev

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Message-ID: <aJ+AP18ucw3Jb6QO@Asurada-Nvidia>
Date: Fri, 15 Aug 2025 11:45:19 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
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Subject: Re: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev_locked()
 helper

On Fri, Aug 15, 2025 at 08:55:28AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@...dia.com>
> > Sent: Tuesday, August 12, 2025 6:59 AM
> > On the other hand, the iommu_get_domain_for_dev() API always returns the
> > group->domain pointer, and several IOMMMU drivers call this API in their
> > attach_dev callback functions to get the currently attached domain for a
> > device, which will be broken for the recovery case mentioned above:
> >  1. core asks the driver to attach dev from blocked to group->domain
> >  2. driver attaches dev from group->domain to group->domain
> 
> the 2nd bullet implies that a driver may skip the operation by noting that
> old domain is same as the new one?

Drivers uses iommu_get_domain_for_dev() to get the "old domain".
But during a reset, it doesn't return the actual old domain that
should be blocked domain but returns the group->domain.

Driver needs the actual domain (blocked) in that case because it
handles the requests from iommu_dev_reset_prepare/done().

> > So, iommu_get_domain_for_dev() should check the gdev flag and return the
> > blocked domain if the flag is set. But the caller of this API could hold
> > the group->mutex already or not, making it difficult to add the lock.
> > 
> > Introduce a new iommu_get_domain_for_dev_locked() helper to be used by
> > those drivers in a context that is already under the protection of the
> > group->mutex, e.g. those attach_dev callback functions. And roll out the
> > new helper to all the existing IOMMU drivers.
> 
> iommu_get_domain_for_dev() is also called outside of attach_dev
> callback functions, e.g. malidp_get_pgsize_bitmap(). and the returned
> info according to the attached domain might be saved in static
> structures, e.g.:
> 
> 	ms->mmu_prefetch_pgsize = malidp_get_pgsize_bitmap(mp);
> 
> would that cause weird issues when blocking domain is returned
> though one may not expect reset to happen at that point?

We aren't changing the iommu_get_domain_for_dev(). So, it should
be used exclusively for functions that are outside group->mutex,
like this one, i.e. they should still get the group->domain v.s.
the blocked domain.

Thanks
Nicolin