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Subject[PATCH v2] iommu/iova: Try harder to allocate from rcache magazine
Date
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>
---

v2: There's no need for a 'proper' stack rotation

drivers/iommu/iova.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 506780084425..bb392fdc7a1b 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -822,12 +822,21 @@ 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;

- return mag->pfns[--mag->size];
+ /* Swap it to pop it */
+ pfn = mag->pfns[i];
+ mag->pfns[i] = mag->pfns[--mag->size];
+
+ return pfn;
}

static void iova_magazine_push(struct iova_magazine *mag, unsigned long pfn)
--
2.13.4.dirty
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