Re: [PATCH v6 1/2] iommu/io-pgtable-arm: Add way to debug pgtable walk

[Mostafa Saleh <smostafa@google.com>]

Hi Rob,

On Wed, Jul 17, 2024 at 09:36:21AM -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>
> Acked-by: Joerg Roedel <jroedel@suse.de>
> 
> ---
>  drivers/iommu/io-pgtable-arm.c | 36 +++++++++++++++++++++++++---------
>  include/linux/io-pgtable.h     | 17 ++++++++++++++++
>  2 files changed, 44 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> index 3d23b924cec1..e70803940b46 100644
> --- a/drivers/iommu/io-pgtable-arm.c
> +++ b/drivers/iommu/io-pgtable-arm.c
> @@ -690,9 +690,11 @@ 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,
> +				 void *_wd)
>  {
> +	struct arm_lpae_io_pgtable_walk_data *wd = _wd;
>  	struct arm_lpae_io_pgtable *data = io_pgtable_ops_to_data(ops);
>  	arm_lpae_iopte pte, *ptep = data->pgd;
>  	int lvl = data->start_level;
> @@ -700,7 +702,7 @@ static phys_addr_t arm_lpae_iova_to_phys(struct io_pgtable_ops *ops,
>  	do {
>  		/* Valid IOPTE pointer? */
>  		if (!ptep)
> -			return 0;
> +			return -ENOENT;
>  
>  		/* Grab the IOPTE we're interested in */
>  		ptep += ARM_LPAE_LVL_IDX(iova, lvl, data);
> @@ -708,22 +710,37 @@ static phys_addr_t arm_lpae_iova_to_phys(struct io_pgtable_ops *ops,
>  
>  		/* Valid entry? */
>  		if (!pte)
> -			return 0;
> +			return -ENOENT;
>  
> -		/* Leaf entry? */
> +		wd->ptes[wd->level++] = pte;
> +
> +		/* 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 0;
> +	return -ENOENT;
> +}
> +
> +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 arm_lpae_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;
>  
> -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 +821,7 @@ 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..df6f6e58310c 100644
> --- a/include/linux/io-pgtable.h
> +++ b/include/linux/io-pgtable.h
> @@ -171,12 +171,28 @@ struct io_pgtable_cfg {
>  	};
>  };
>  
> +/**
> + * struct arm_lpae_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 arm_lpae_io_pgtable_walk_data {
> +	u64 ptes[4];

I don’t see a reason to save the whole walk for iova_to_phys,
I see that the DRM driver uses those next, but I am worried that
won’t scale, a callback mechanism sounds better.

Also, there is a page table walker recently added to io-pagtable-arm,
for dirty bit tracking:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/iommu/io-pgtable-arm.c?id=4fe88fd8b4aecb7f9680bf898811db76b94095a9

I’d suggest consolidating those into one walker where each caller
has its own logic in a callback.

Thanks,
Mostafa

> +	int level;
> +};
> +
>  /**
>   * struct io_pgtable_ops - Page table manipulation API for IOMMU drivers.
>   *
>   * @map_pages:    Map a physically contiguous range of pages of the same size.
>   * @unmap_pages:  Unmap a range of virtually contiguous pages of the same size.
>   * @iova_to_phys: Translate iova to physical address.
> + * @pgtable_walk: (optional) Perform a page table walk for a given iova.  The
> + *                type for the wd parameter is specific to pgtable type, as
> + *                the PTE size and number of levels differs per pgtable type.
>   *
>   * These functions map directly onto the iommu_ops member functions with
>   * the same names.
> @@ -190,6 +206,7 @@ struct io_pgtable_ops {
>  			      struct iommu_iotlb_gather *gather);
>  	phys_addr_t (*iova_to_phys)(struct io_pgtable_ops *ops,
>  				    unsigned long iova);
> +	int (*pgtable_walk)(struct io_pgtable_ops *ops, unsigned long iova, void *wd);
>  	int (*read_and_clear_dirty)(struct io_pgtable_ops *ops,
>  				    unsigned long iova, size_t size,
>  				    unsigned long flags,
> -- 
> 2.45.2
>