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

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Date: Tue, 25 Jun 2024 12:27:04 +0100
From: Will Deacon <will@...nel.org>
To: Rob Clark <robdclark@...il.com>
Cc: dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
	freedreno@...ts.freedesktop.org, Rob Clark <robdclark@...omium.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 08:37:26AM -0700, Rob Clark wrote:
> 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.
> 
> fwiw, I passed it as a void* to avoid the pte size.. although I guess
> it could be a union of all the possible pte types

Can you just get away with a u64?

Will