[PATCH] iommu: Improve the performance for direct_mapping

[Yong Wu]

On Mon, 2020-11-23 at 12:32 +0000, Will Deacon wrote:
> On Fri, Nov 20, 2020 at 05:06:28PM +0800, Yong Wu wrote:
> > Currently direct_mapping always use the smallest pgsize which is SZ_4K
> > normally to mapping. This is unnecessary. we could gather the size, and
> > call iommu_map then, iommu_map could decide how to map better with the
> > just right pgsize.
> > 
> > From the original comment, we should take care overlap, otherwise,
> > iommu_map may return -EEXIST. In this overlap case, we should map the
> > previous region before overlap firstly. then map the left part.
> > 
> > Each a iommu device will call this direct_mapping when its iommu
> > initialize, This patch is effective to improve the boot/initialization
> > time especially while it only needs level 1 mapping.
> > 
> > Signed-off-by: Anan Sun <anan.sun at mediatek.com>
> > Signed-off-by: Yong Wu <yong.wu at mediatek.com>
> > ---
> >  drivers/iommu/iommu.c | 20 ++++++++++++++++++--
> >  1 file changed, 18 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index df87c8e825f7..854a8fcb928d 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -737,6 +737,7 @@ static int iommu_create_device_direct_mappings(struct iommu_group *group,
> >  	/* We need to consider overlapping regions for different devices */
> >  	list_for_each_entry(entry, &mappings, list) {
> >  		dma_addr_t start, end, addr;
> > +		size_t unmapped_sz = 0;
> 
> I think "unmapped" is the wrong word here, as this variable actually
> represents the amount we want to map! I suggest "map_size" instead.
> 
> >  		if (domain->ops->apply_resv_region)
> >  			domain->ops->apply_resv_region(dev, domain, entry);
> > @@ -752,10 +753,25 @@ static int iommu_create_device_direct_mappings(struct iommu_group *group,
> >  			phys_addr_t phys_addr;
> >  
> >  			phys_addr = iommu_iova_to_phys(domain, addr);
> > -			if (phys_addr)
> > +			if (phys_addr == 0) {
> > +				unmapped_sz += pg_size; /* Gather the size. */
> >  				continue;
> > +			}
> >  
> > -			ret = iommu_map(domain, addr, addr, pg_size, entry->prot);
> > +			if (unmapped_sz) {
> > +				/* Map the region before the overlap. */
> > +				ret = iommu_map(domain, start, start,
> > +						unmapped_sz, entry->prot);
> > +				if (ret)
> > +					goto out;
> > +				start += unmapped_sz;
> 
> I think it's a bit confusing to update start like this. Can we call
> iommu_map(domain, addr - map_size, addr - map_size, map_size, entry->prot)
> instead?
> 
> > +				unmapped_sz = 0;
> > +			}
> > +			start += pg_size;
> > +		}
> > +		if (unmapped_sz) {
> > +			ret = iommu_map(domain, start, start, unmapped_sz,
> > +					entry->prot);
> 
> Can you avoid this hunk by changing your loop check to something like:
> 
> 	if (!phys_addr) {
> 		map_size += pg_size;
> 		if (addr + pg_size < end)
> 			continue;
> 	}

Thanks for your quick review. I have fixed and tested it. the patch is
simple. I copy it here. Is this readable for you now?


--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -737,6 +737,7 @@ static int
iommu_create_device_direct_mappings(struct iommu_group *group,
 	/* We need to consider overlapping regions for different devices */
 	list_for_each_entry(entry, &mappings, list) {
 		dma_addr_t start, end, addr;
+		size_t map_size = 0;
 
 		if (domain->ops->apply_resv_region)
 			domain->ops->apply_resv_region(dev, domain, entry);
@@ -752,12 +753,21 @@ static int
iommu_create_device_direct_mappings(struct iommu_group *group,
 			phys_addr_t phys_addr;
 
 			phys_addr = iommu_iova_to_phys(domain, addr);
-			if (phys_addr)
-				continue;
+			if (!phys_addr) {
+				map_size += pg_size;
+				if (addr + pg_size < end)
+					continue;
+				else
+					addr += pg_size; /*Point to End */
+			}
 
-			ret = iommu_map(domain, addr, addr, pg_size, entry->prot);
-			if (ret)
-				goto out;
+			if (map_size) {
+				ret = iommu_map(domain, addr - map_size, addr - map_size,
+						map_size, entry->prot);
+				if (ret)
+					goto out;
+				map_size = 0;
+			}
 		}

> 
> Will