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

[Will Deacon <will@kernel.org>]

On Tue, May 20, 2025 at 02:06:09PM +0100, Robin Murphy wrote:
> On 2025-05-20 12:31 pm, Will Deacon wrote:
> > 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:
> > > > > 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.
> 
> Hmm, just a nit on reflection, how about fixing up the name to just
> IO_PGTABLE_QUIRK_NO_WARN? Given that it's already quite long, and we have a
> well-established DMA_ATTR_NO_WARN with equivalent semantics over in the DMA
> API.

Sure, I'll do that now...

Will