iommu/iova: Try harder to allocate from rcache magazine

When devices with different DMA masks are using the same domain, or for
PCI devices where we usually try a speculative 32-bit allocation first,
there is a fair possibility that the top PFN of the rcache stack at any
given time may be unsuitable for the lower limit, prompting a fallback
to allocating anew from the rbtree. Consequently, we may end up
artifically increasing pressure on the 32-bit IOVA space as unused IOVAs
accumulate lower down in the rcache stacks, while callers with 32-bit
masks also impose unnecessary rbtree overhead.

In such cases, let's try a bit harder to satisfy the allocation locally
first - scanning the whole stack should still be relatively inexpensive.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 15ff303..b0ca236 100644 (file)
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -821,12 +821,21 @@ static bool iova_magazine_empty(struct iova_magazine *mag)
 static unsigned long iova_magazine_pop(struct iova_magazine *mag,
                                       unsigned long limit_pfn)
 {
+       int i;
+       unsigned long pfn;
+
        BUG_ON(iova_magazine_empty(mag));
 
-       if (mag->pfns[mag->size - 1] > limit_pfn)
-               return 0;
+       /* Only fall back to the rbtree if we have no suitable pfns at all */
+       for (i = mag->size - 1; mag->pfns[i] > limit_pfn; i--)
+               if (i == 0)
+                       return 0;
+
+       /* Swap it to pop it */
+       pfn = mag->pfns[i];
+       mag->pfns[i] = mag->pfns[--mag->size];
 
-       return mag->pfns[--mag->size];
+       return pfn;
 }
 
 static void iova_magazine_push(struct iova_magazine *mag, unsigned long pfn)