Re: [PATCH] iommu/vt-d: reduce extra first level entry in iommu->domains

[Robin Murphy]

On 25/05/16 00:06, Wei Yang wrote:
> Hi, Joerg
>
> Not sure whether you think this calculation is correct.
>
> If I missed something for this " + 1" in your formula, I am glad to hear your
> explanation. So that I could learn something from you :-)

I'm not familiar enough with this aspect of the driver to confirm 
whether the change is appropriate or not, but it does seem worth noting 
that using DIV_ROUND_UP would be an even neater approach.

Robin.

> Have a good day~
>
> On Sat, May 21, 2016 at 02:41:51AM +0000, Wei Yang wrote:
> > In commit <8bf478163e69> ("iommu/vt-d: Split up iommu->domains array"), it
> > it splits iommu->domains in two levels. Each first level contains 256
> > entries of second level. In case of the ndomains is exact a multiple of
> > 256, it would have one more extra first level entry for current
> > implementation.
> >
> > This patch refines this calculation to reduce the extra first level entry.
> >
> > Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
> > ---
> > drivers/iommu/intel-iommu.c |    4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
> > index e3061d3..2204ca4 100644
> > --- a/drivers/iommu/intel-iommu.c
> > +++ b/drivers/iommu/intel-iommu.c
> > @@ -1634,7 +1634,7 @@ static int iommu_init_domains(struct intel_iommu *iommu)
> > 		return -ENOMEM;
> > 	}
> >
> > -	size = ((ndomains >> 8) + 1) * sizeof(struct dmar_domain **);
> > +	size = (ALIGN(ndomains, 256) >> 8) * sizeof(struct dmar_domain **);
> > 	iommu->domains = kzalloc(size, GFP_KERNEL);
> >
> > 	if (iommu->domains) {
> > @@ -1699,7 +1699,7 @@ static void disable_dmar_iommu(struct intel_iommu *iommu)
> > static void free_dmar_iommu(struct intel_iommu *iommu)
> > {
> > 	if ((iommu->domains) && (iommu->domain_ids)) {
> > -		int elems = (cap_ndoms(iommu->cap) >> 8) + 1;
> > +		int elems = ALIGN(cap_ndoms(iommu->cap), 256) >> 8;
> > 		int i;
> >
> > 		for (i = 0; i < elems; i++)
> > --
> > 1.7.9.5