linux-kernel - Re: [PATCH v4 1/2] iommu/io-pgtable-arm: Add way to debug pgtable walk

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Message-ID: <CAJs_Fx5jWJTUjaw1X0htzsmBVo_E6kkzOFDFMM5gazata5u0Vg@mail.gmail.com>
Date: Wed, 26 Jun 2024 11:50:59 -0700
From: Rob Clark <robdclark@...omium.org>
To: Will Deacon <will@...nel.org>
Cc: Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org, 
	linux-arm-msm@...r.kernel.org, freedreno@...ts.freedesktop.org, 
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>, Jason Gunthorpe <jgg@...pe.ca>, 
	Boris Brezillon <boris.brezillon@...labora.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 1/2] iommu/io-pgtable-arm: Add way to debug pgtable walk

On Mon, Jun 24, 2024 at 8:14 AM Will Deacon <will@...nel.org> wrote:
>
> On Thu, May 23, 2024 at 10:52:21AM -0700, Rob Clark wrote:
> > From: Rob Clark <robdclark@...omium.org>
> >
> > Add an io-pgtable method to walk the pgtable returning the raw PTEs that
> > would be traversed for a given iova access.
> >
> > Signed-off-by: Rob Clark <robdclark@...omium.org>
> > ---
> >  drivers/iommu/io-pgtable-arm.c | 51 ++++++++++++++++++++++++++++------
> >  include/linux/io-pgtable.h     |  4 +++
> >  2 files changed, 46 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> > index f7828a7aad41..f47a0e64bb35 100644
> > --- a/drivers/iommu/io-pgtable-arm.c
> > +++ b/drivers/iommu/io-pgtable-arm.c
> > @@ -693,17 +693,19 @@ static size_t arm_lpae_unmap_pages(struct io_pgtable_ops *ops, unsigned long iov
> >                               data->start_level, ptep);
> >  }
> >
> > -static phys_addr_t arm_lpae_iova_to_phys(struct io_pgtable_ops *ops,
> > -                                      unsigned long iova)
> > +static int arm_lpae_pgtable_walk(struct io_pgtable_ops *ops, unsigned long iova,
> > +                     int (*cb)(void *cb_data, void *pte, int level),
> > +                     void *cb_data)
> >  {
> >       struct arm_lpae_io_pgtable *data = io_pgtable_ops_to_data(ops);
> >       arm_lpae_iopte pte, *ptep = data->pgd;
> >       int lvl = data->start_level;
> > +     int ret;
> >
> >       do {
> >               /* Valid IOPTE pointer? */
> >               if (!ptep)
> > -                     return 0;
> > +                     return -EFAULT;
>
> nit: -ENOENT might be a little better, as we're only checking against a
> NULL entry rather than strictly any faulting entry.
>
> >               /* Grab the IOPTE we're interested in */
> >               ptep += ARM_LPAE_LVL_IDX(iova, lvl, data);
> > @@ -711,22 +713,52 @@ static phys_addr_t arm_lpae_iova_to_phys(struct io_pgtable_ops *ops,
> >
> >               /* Valid entry? */
> >               if (!pte)
> > -                     return 0;
> > +                     return -EFAULT;
>
> Same here (and at the end of the function).
>
> > +
> > +             ret = cb(cb_data, &pte, lvl);
>
> Since pte is on the stack, rather than pointing into the actual pgtable,
> I think it would be clearer to pass it by value to the callback.
>
> > +             if (ret)
> > +                     return ret;
> >
> > -             /* Leaf entry? */
> > +             /* Leaf entry?  If so, we've found the translation */
> >               if (iopte_leaf(pte, lvl, data->iop.fmt))
> > -                     goto found_translation;
> > +                     return 0;
> >
> >               /* Take it to the next level */
> >               ptep = iopte_deref(pte, data);
> >       } while (++lvl < ARM_LPAE_MAX_LEVELS);
> >
> >       /* Ran out of page tables to walk */
> > +     return -EFAULT;
> > +}
> > +
> > +struct iova_to_phys_walk_data {
> > +     arm_lpae_iopte pte;
> > +     int level;
> > +};
>
> Expanding a little on Robin's suggestion, why don't we drop this structure
> in favour of something more generic:
>
>         struct arm_lpae_walk_data {
>                 arm_lpae_iopte ptes[ARM_LPAE_MAX_LEVELS];
>         };
>
> and then do something in the walker like:
>
>         if (cb && !cb(pte, lvl))
>                 walk_data->ptes[lvl] = pte;
>

So thinking about this some more... if I use a walk_data struct to
return the PTEs, I can just get rid of the callback entirely.  That
ends up looking more like my first version.   The callback taking
void* was mainly to avoid coding the PTE size in the generic
io_pgtable interface.  But if we just go with u64, because that is the
biggest PTE size we need to deal with, then it all gets simpler.  (The
callback was actually a semi-awkward interface to use from drm/msm.)

BR,
-R

> which could return the physical address at the end, if it reaches a leaf
> entry. That way arm_lpae_iova_to_phys() is just passing a NULL callback
> to the walker and your debug callback just needs to return 0 (i.e. the
> callback is basically just saying whether or not to continue the walk).
>
> Will