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

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Date: Mon, 24 Jun 2024 16:14:01 +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 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;

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