Re: [PATCH v4 05/40] iommu/io-pgtable-arm: Add quirk to quiet WARN_ON()

[Will Deacon <will@kernel.org>]

On Thu, May 15, 2025 at 07:48:39AM -0700, Rob Clark wrote:
> On Thu, May 15, 2025 at 7:33 AM Will Deacon <will@kernel.org> wrote:
> >
> > On Wed, May 14, 2025 at 10:53:19AM -0700, Rob Clark wrote:
> > > From: Rob Clark <robdclark@chromium.org>
> > >
> > > In situations where mapping/unmapping sequence can be controlled by
> > > userspace, attempting to map over a region that has not yet been
> > > unmapped is an error.  But not something that should spam dmesg.
> > >
> > > Now that there is a quirk, we can also drop the selftest_running
> > > flag, and use the quirk instead for selftests.
> > >
> > > Signed-off-by: Rob Clark <robdclark@chromium.org>
> > > Acked-by: Robin Murphy <robin.murphy@arm.com>
> > > Signed-off-by: Rob Clark <robdclark@chromium.org>
> > > ---
> > >  drivers/iommu/io-pgtable-arm.c | 27 ++++++++++++++-------------
> > >  include/linux/io-pgtable.h     |  8 ++++++++
> > >  2 files changed, 22 insertions(+), 13 deletions(-)
> >
> > [...]
> >
> > > diff --git a/include/linux/io-pgtable.h b/include/linux/io-pgtable.h
> > > index bba2a51c87d2..639b8f4fb87d 100644
> > > --- a/include/linux/io-pgtable.h
> > > +++ b/include/linux/io-pgtable.h
> > > @@ -88,6 +88,13 @@ struct io_pgtable_cfg {
> > >        *
> > >        * IO_PGTABLE_QUIRK_ARM_HD: Enables dirty tracking in stage 1 pagetable.
> > >        * IO_PGTABLE_QUIRK_ARM_S2FWB: Use the FWB format for the MemAttrs bits
> > > +      *
> > > +      * IO_PGTABLE_QUIRK_NO_WARN_ON: Do not WARN_ON() on conflicting
> > > +      *      mappings, but silently return -EEXISTS.  Normally an attempt
> > > +      *      to map over an existing mapping would indicate some sort of
> > > +      *      kernel bug, which would justify the WARN_ON().  But for GPU
> > > +      *      drivers, this could be under control of userspace.  Which
> > > +      *      deserves an error return, but not to spam dmesg.
> > >        */
> > >       #define IO_PGTABLE_QUIRK_ARM_NS                 BIT(0)
> > >       #define IO_PGTABLE_QUIRK_NO_PERMS               BIT(1)
> > > @@ -97,6 +104,7 @@ struct io_pgtable_cfg {
> > >       #define IO_PGTABLE_QUIRK_ARM_OUTER_WBWA         BIT(6)
> > >       #define IO_PGTABLE_QUIRK_ARM_HD                 BIT(7)
> > >       #define IO_PGTABLE_QUIRK_ARM_S2FWB              BIT(8)
> > > +     #define IO_PGTABLE_QUIRK_NO_WARN_ON             BIT(9)
> >
> > This feels a bit fragile to me:
> >   * IOMMU-API users of io-pgtable shouldn't be passing this quirk
> >     but might end up doing so to paper over driver bugs.
> >
> >   * Low-level users of io-pgtable who expose page-table operations to
> >     userspace need to pass the quirk, but might well not bother because
> >     well-behaved userspace doesn't trigger the warning.
> >
> > So overall, it's all a bit unsatisfactory. Is there a way we could have
> > the warnings only when invoked via the IOMMU API?
> 
> iommu drivers _not_ setting this flag seems like a good way to achieve that ;-)
> 
> The alternative is to move the warns to the iommu driver... but they
> could just as easily remove the WARN_ON()s as they could set the
> NO_WARN_ON quirk, so :shrug:?

Bah, I also don't have a good idea to improve this, so I guess I'll take
what you have for now.

Will