Re: [RFC PATCH 3/6] iommu: Use bitmap to calculate page size in iommu_pgsize()

[isaacm@codeaurora.org]

On 2021-04-01 09:47, Will Deacon wrote:
> 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>
> ---
>  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..bcd623862bf9 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(__fls(size), 0);
I've fixed this in the latest RFC for the iommu_map/unmap optimization 
patches,
but for the sake of completeness: I think this should be GENMASK_ULL, in 
case
__fls(size) >= 32.

Thank you for these patches, by the way. I've looked through them and 
they
make sense/seem correct. I've integrated them into the latest RFC: 
https://lore.kernel.org/linux-iommu/20210402013452.4013-1-isaacm@codeaurora.org/T/#t.

Thanks,
Isaac
> 
> -	/* need to consider alignment requirements ? */
> -	if (likely(addr_merge)) {
> -		/* Max page size allowed by address */
> -		unsigned int align_pgsize_idx = __ffs(addr_merge);
> -		pgsize_idx = min(pgsize_idx, align_pgsize_idx);
> -	}
> -
> -	/* build a mask of acceptable page sizes */
> -	pgsize = (1UL << (pgsize_idx + 1)) - 1;
> -
> -	/* throw away page sizes not supported by the hardware */
> -	pgsize &= domain->pgsize_bitmap;
> +	/* Constrain the page sizes further based on the maximum alignment */
> +	if (likely(addr_merge))
> +		pgsizes &= GENMASK(__ffs(addr_merge), 0);
> 
> -	/* make sure we're still sane */
> -	BUG_ON(!pgsize);
> +	/* Make sure we have at least one suitable page size */
> +	BUG_ON(!pgsizes);
> 
> -	/* pick the biggest page */
> -	pgsize_idx = __fls(pgsize);
> -	pgsize = 1UL << pgsize_idx;
> +	/* Pick the biggest page size remaining */
> +	pgsize_idx = __fls(pgsizes);
> +	pgsize = BIT(pgsize_idx);
> 
>  	return pgsize;
>  }
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