linux-kernel - Re: [PATCH v5 1/2] iommu/io-pgtable-arm: Add way to debug pgtable walk

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Message-ID: <CAF6AEGsgN8O2eJGqcJm1UaPzV2rWSXskAc+A8uk0mVbsj8Wm8A@mail.gmail.com>
Date: Wed, 3 Jul 2024 09:18:43 -0700
From: Rob Clark <robdclark@...il.com>
To: Will Deacon <will@...nel.org>
Cc: iommu@...ts.linux.dev, linux-arm-msm@...r.kernel.org, 
	Robin Murphy <robin.murphy@....com>, Rob Clark <robdclark@...omium.org>, 
	Joerg Roedel <joro@...tes.org>, Jason Gunthorpe <jgg@...pe.ca>, Steven Price <steven.price@....com>, 
	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 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/2] iommu/io-pgtable-arm: Add way to debug pgtable walk

On Wed, Jul 3, 2024 at 8:02 AM Will Deacon <will@...nel.org> wrote:
>
> Hi Rob,
>
> On Wed, Jun 26, 2024 at 01:40:26PM -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 | 34 +++++++++++++++++++++++++---------
> >  include/linux/io-pgtable.h     | 16 ++++++++++++++++
> >  2 files changed, 41 insertions(+), 9 deletions(-)
>
> Non-technical question, but with patch 2/2 being drm-specific, how do
> you plan to get this merged this once it's finalised? I can take this
> part via the IOMMU tree?

I guess if need be, I could merge the drm part only after the iommu
part lands.  We've lived with an earlier iteration of these series as
downstream patches in the CrOS kernel for this long, it isn't the end
of the world if it takes a bit longer

> > +static phys_addr_t arm_lpae_iova_to_phys(struct io_pgtable_ops *ops,
> > +                                      unsigned long iova)
> > +{
> > +     struct arm_lpae_io_pgtable *data = io_pgtable_ops_to_data(ops);
> > +     struct io_pgtable_walk_data wd = {};
> > +     int ret, lvl;
> > +
> > +     ret = arm_lpae_pgtable_walk(ops, iova, &wd);
> > +     if (ret)
> > +             return 0;
> > +
> > +     lvl = wd.level + data->start_level;
>
> nit, but the level is architectural so I think we should initialise
> wd.level to data->start_level instead.

Hmm, I wanted to use wd.level to get the index of the last entry in
wd.ptes.  Perhaps I should just call it something other than 'level'
instead?

> >
> > -found_translation:
> >       iova &= (ARM_LPAE_BLOCK_SIZE(lvl, data) - 1);
> > -     return iopte_to_paddr(pte, data) | iova;
> > +     return iopte_to_paddr(wd.ptes[wd.level - 1], data) | iova;
> >  }
> >
> >  static void arm_lpae_restrict_pgsizes(struct io_pgtable_cfg *cfg)
> > @@ -804,6 +819,7 @@ arm_lpae_alloc_pgtable(struct io_pgtable_cfg *cfg)
> >               .map_pages      = arm_lpae_map_pages,
> >               .unmap_pages    = arm_lpae_unmap_pages,
> >               .iova_to_phys   = arm_lpae_iova_to_phys,
> > +             .pgtable_walk   = arm_lpae_pgtable_walk,
> >       };
> >
> >       return data;
> > diff --git a/include/linux/io-pgtable.h b/include/linux/io-pgtable.h
> > index 86cf1f7ae389..4d696724c7da 100644
> > --- a/include/linux/io-pgtable.h
> > +++ b/include/linux/io-pgtable.h
> > @@ -171,12 +171,26 @@ struct io_pgtable_cfg {
> >       };
> >  };
> >
> > +/**
> > + * struct io_pgtable_walk_data - information from a pgtable walk
> > + *
> > + * @ptes:     The recorded PTE values from the walk
> > + * @level:    The level of the last PTE
> > + *
> > + * @level also specifies the last valid index in @ptes
> > + */
> > +struct io_pgtable_walk_data {
> > +     u64 ptes[4];
> > +     int level;
> > +};
>
> I wonder if we can do better than hardcoding the '4' here? I wouldn't be
> surprised if this doesn't work, but could we do something along the
> lines of:
>
> struct io_pgtable_walk_data {
>         int level;
>         int num_levels;
>         u64 ptes[] __counted_by(num_levels);
> };
>
> and then have the Arm (LPAE)-specific code wrap that in a private
> structure:
>
> struct arm_lpae_io_pgtable_walk_data {
>         struct io_pgtable_walk_data data;
>         u64 ptes[ARM_LPAE_MAX_LEVELS];
> };
>
> which is used by the walker?

I guess we could just make the walk_data fxn ptr arg a void* and
rename io_pgtable_walk_data -> io_lpae_pgtable_walk_data?  I'm not
sure how hard to try to make this interface generic when I think it is
probably only going to be used by code that knows what pgtable format
is used.  It's kinda the same question about what type to use for the
pte.  Maybe the answer is just "it's pgtable format dependent"?

BR,
-R