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

[Will Deacon]

On Thu, Apr 01, 2021 at 06:39:35PM -0700, isaacm at codeaurora.org wrote:
> 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 at 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.

Hmm, but 'size' is a size_t; which architectures have sizeof(size_t) >
sizeof(unsigned long)?

> > -	/* 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);

This one looks like more of an issue, though, as addr_merge is a
phys_addr_t, which certainly can be 64-bit where unsigned long is 32-bit
(e.g. Armv7 + LPAE)

Rather than make everything _ULL, please can you just do it where we're
actually using the larger types?

Will