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

Robin Murphy
2024-06-17T15:13:45Z

On 23/05/2024 6:52 pm, 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.

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@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;
>
> /* 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,