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

Will Deacon
2024-06-24T15:14:07Z

On Thu, May 23, 2024 at 10:52:21AM -0700, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.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@chromium.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;

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