Re: [RFC PATCH v3 05/12] iommu: Use bitmap to calculate page size in iommu_pgsize()

[Will Deacon <will@kernel.org>]

On Mon, Apr 05, 2021 at 12:11:05PM -0700, Isaac J. Manjarres wrote:
> From: Will Deacon <will@kernel.org>
> 
> Avoid the potential for shifting values by amounts greater than the
> width of their type by using a bitmap to compute page size in
> iommu_pgsize().
> 
> Signed-off-by: Will Deacon <will@kernel.org>
> Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
> ---
>  drivers/iommu/iommu.c | 31 ++++++++++++-------------------
>  1 file changed, 12 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index d0b0a15dba84..9006397b6604 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -8,6 +8,7 @@
>  
>  #include <linux/device.h>
>  #include <linux/kernel.h>
> +#include <linux/bits.h>
>  #include <linux/bug.h>
>  #include <linux/types.h>
>  #include <linux/init.h>
> @@ -2360,30 +2361,22 @@ static size_t iommu_pgsize(struct iommu_domain *domain,
>  			   unsigned long addr_merge, size_t size)
>  {
>  	unsigned int pgsize_idx;
> +	unsigned long pgsizes;
>  	size_t pgsize;
>  
> -	/* Max page size that still fits into 'size' */
> -	pgsize_idx = __fls(size);
> +	/* Page sizes supported by the hardware and small enough for @size */
> +	pgsizes = domain->pgsize_bitmap & GENMASK_ULL(__fls(size), 0);

See my comments on the other thread, but I don't think it's necessary to use
the _ULL versions everywhere.

Will

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