Re: [PATCH 3/4] iommu/amd: Do not sync on page size changes

[Robin Murphy <robin.murphy@arm.com>]

On 2021-05-02 07:59, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> Some IOMMU architectures perform invalidations regardless of the page
> size. In such architectures there is no need to sync when the page size
> changes or to regard pgsize when making interim flush in
> iommu_iotlb_gather_add_page().
> 
> Add a "ignore_gather_pgsize" property for each IOMMU-ops to decide
> whether gather's pgsize is tracked and triggers a flush.

I've objected before[1], and I'll readily object again ;)

I still think it's very silly to add a bunch of indirection all over the 
place to make a helper function not do the main thing it's intended to 
help with. If you only need trivial address gathering then it's far 
simpler to just implement trivial address gathering. I suppose if you 
really want to you could factor out another helper to share the 5 lines 
of code which ended up in mtk-iommu (see commit f21ae3b10084).

Robin.

[1] 
https://lore.kernel.org/linux-iommu/49bae447-d662-e6cf-7500-ab78e3b75dc4@arm.com/

> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Jiajun Cao <caojiajun@vmware.com>
> Cc: iommu@lists.linux-foundation.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
>   drivers/iommu/amd/iommu.c | 1 +
>   include/linux/iommu.h     | 3 ++-
>   2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
> index b8cabbbeed71..1849b53f2149 100644
> --- a/drivers/iommu/amd/iommu.c
> +++ b/drivers/iommu/amd/iommu.c
> @@ -2215,6 +2215,7 @@ const struct iommu_ops amd_iommu_ops = {
>   	.put_resv_regions = generic_iommu_put_resv_regions,
>   	.is_attach_deferred = amd_iommu_is_attach_deferred,
>   	.pgsize_bitmap	= AMD_IOMMU_PGSIZES,
> +	.ignore_gather_pgsize = true,
>   	.flush_iotlb_all = amd_iommu_flush_iotlb_all,
>   	.iotlb_sync = amd_iommu_iotlb_sync,
>   	.def_domain_type = amd_iommu_def_domain_type,
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 32d448050bf7..1fb2695418e9 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -284,6 +284,7 @@ struct iommu_ops {
>   	int (*def_domain_type)(struct device *dev);
>   
>   	unsigned long pgsize_bitmap;
> +	bool ignore_gather_pgsize;
>   	struct module *owner;
>   };
>   
> @@ -508,7 +509,7 @@ static inline void iommu_iotlb_gather_add_page(struct iommu_domain *domain,
>   	 * a different granularity, then sync the TLB so that the gather
>   	 * structure can be rewritten.
>   	 */
> -	if (gather->pgsize != size ||
> +	if ((gather->pgsize != size && !domain->ops->ignore_gather_pgsize) ||
>   	    end + 1 < gather->start || start > gather->end + 1) {
>   		if (gather->pgsize)
>   			iommu_iotlb_sync(domain, gather);
>