linux-kernel - Re: [PATCH v4 05/40] iommu/io-pgtable-arm: Add quirk to quiet WARN

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Message-ID: <CAF6AEGsnOD8fZmTXAEZZNrdK-NXdUJF51s51EhYQ6Ed7dCFM0A@mail.gmail.com>
Date: Thu, 15 May 2025 07:48:39 -0700
From: Rob Clark <robdclark@...il.com>
To: Will Deacon <will@...nel.org>
Cc: dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org, 
	linux-arm-msm@...r.kernel.org, Connor Abbott <cwabbott0@...il.com>, 
	Rob Clark <robdclark@...omium.org>, Robin Murphy <robin.murphy@....com>, 
	Joerg Roedel <joro@...tes.org>, Jason Gunthorpe <jgg@...pe.ca>, Nicolin Chen <nicolinc@...dia.com>, 
	Kevin Tian <kevin.tian@...el.com>, Joao Martins <joao.m.martins@...cle.com>, 
	"moderated list:ARM SMMU DRIVERS" <linux-arm-kernel@...ts.infradead.org>, 
	"open list:IOMMU SUBSYSTEM" <iommu@...ts.linux.dev>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 05/40] iommu/io-pgtable-arm: Add quirk to quiet WARN_ON()

On Thu, May 15, 2025 at 7:33 AM Will Deacon <will@...nel.org> wrote:
>
> On Wed, May 14, 2025 at 10:53:19AM -0700, Rob Clark wrote:
> > From: Rob Clark <robdclark@...omium.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@...omium.org>
> > Acked-by: Robin Murphy <robin.murphy@....com>
> > Signed-off-by: Rob Clark <robdclark@...omium.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:?

BR,
-R