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

[Robin Murphy <robin.murphy@xxxxxxx>]

On 23/05/2024 6:52 pm, Rob Clark wrote:
> From: Rob Clark <robdclark@xxxxxxxxxxxx>
>
> Add an io-pgtable method to walk the pgtable returning the raw PTEs that
> would be traversed for a given iova access.

Have to say I'm a little torn here - with my iommu-dma hat on I'm not 
super enthusiastic about adding any more overhead to iova_to_phys, but 
in terms of maintaining io-pgtable I do like the overall shape of the 
implementation...

Will, how much would you hate a compromise of inlining iova_to_phys as 
the default walk behaviour if cb is NULL? :)

That said, looking at the unmap figures for dma_map_benchmark on a 
Neoverse N1, any difference I think I see is still well within the 
noise, so maybe a handful of extra indirect calls isn't really enough to 
worry about?

Cheers,
Robin.

> Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> ---
>   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;
>   
>   		/* 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;
> +
> +		ret = cb(cb_data, &pte, lvl);
> +		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;
> +};
> +
> +static int iova_to_phys_walk_cb(void *cb_data, void *pte, int level)
> +{
> +	struct iova_to_phys_walk_data *d = cb_data;
> +
> +	d->pte = *(arm_lpae_iopte *)pte;
> +	d->level = level;
> +
>   	return 0;
> +}
> +
> +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 iova_to_phys_walk_data d;
> +	int ret;
> +
> +	ret = arm_lpae_pgtable_walk(ops, iova, iova_to_phys_walk_cb, &d);
> +	if (ret)
> +		return 0;
>   
> -found_translation:
> -	iova &= (ARM_LPAE_BLOCK_SIZE(lvl, data) - 1);
> -	return iopte_to_paddr(pte, data) | iova;
> +	iova &= (ARM_LPAE_BLOCK_SIZE(d.level, data) - 1);
> +	return iopte_to_paddr(d.pte, data) | iova;
>   }
>   
>   static void arm_lpae_restrict_pgsizes(struct io_pgtable_cfg *cfg)
> @@ -807,6 +839,7 @@ 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..261b48af068a 100644
> --- a/include/linux/io-pgtable.h
> +++ b/include/linux/io-pgtable.h
> @@ -177,6 +177,7 @@ struct io_pgtable_cfg {
>    * @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.
>    *
>    * These functions map directly onto the iommu_ops member functions with
>    * the same names.
> @@ -190,6 +191,9 @@ 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,
> +			    int (*cb)(void *cb_data, void *pte, int level),
> +			    void *cb_data);
>   	int (*read_and_clear_dirty)(struct io_pgtable_ops *ops,
>   				    unsigned long iova, size_t size,
>   				    unsigned long flags,