iommu/iova: Make rcache flush optional on IOVA allocation failure

Since IOVA allocation failure is not unusual case we need to flush
CPUs' rcache in hope we will succeed in next round.

However, it is useful to decide whether we need rcache flush step because
of two reasons:
- Not scalability. On large system with ~100 CPUs iterating and flushing
  rcache for each CPU becomes serious bottleneck so we may want to defer it.
- free_cpu_cached_iovas() does not care about max PFN we are interested in.
  Thus we may flush our rcaches and still get no new IOVA like in the
  commonly used scenario:

    if (dma_limit > DMA_BIT_MASK(32) && dev_is_pci(dev))
        iova = alloc_iova_fast(iovad, iova_len, DMA_BIT_MASK(32) >> shift);

    if (!iova)
        iova = alloc_iova_fast(iovad, iova_len, dma_limit >> shift);

   1. First alloc_iova_fast() call is limited to DMA_BIT_MASK(32) to get
      PCI devices a SAC address
   2. alloc_iova() fails due to full 32-bit space
   3. rcaches contain PFNs out of 32-bit space so free_cpu_cached_iovas()
      throws entries away for nothing and alloc_iova() fails again
   4. Next alloc_iova_fast() call cannot take advantage of rcache since we
      have just defeated caches. In this case we pick the slowest option
      to proceed.

This patch reworks flushed_rcache local flag to be additional function
argument instead and control rcache flush step. Also, it updates all users
to do the flush as the last chance.

Signed-off-by: Tomasz Nowicki <Tomasz.Nowicki@caviumnetworks.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Nate Watterson <nwatters@codeaurora.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

1 files changed, 6 insertions, 5 deletions

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 3aee64b99df1..84bda3a4dafc 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -395,14 +395,15 @@ EXPORT_SYMBOL_GPL(free_iova);
  * @iovad: - iova domain in question
  * @size: - size of page frames to allocate
  * @limit_pfn: - max limit address
+ * @flush_rcache: - set to flush rcache on regular allocation failure
  * This function tries to satisfy an iova allocation from the rcache,
- * and falls back to regular allocation on failure.
+ * and falls back to regular allocation on failure. If regular allocation
+ * fails too and the flush_rcache flag is set then the rcache will be flushed.
 */
 unsigned long
 alloc_iova_fast(struct iova_domain *iovad, unsigned long size,
-		unsigned long limit_pfn)
+		unsigned long limit_pfn, bool flush_rcache)
 {
-	bool flushed_rcache = false;
 	unsigned long iova_pfn;
 	struct iova *new_iova;
 
@@ -415,11 +416,11 @@ retry:
 	if (!new_iova) {
 		unsigned int cpu;
 
-		if (flushed_rcache)
+		if (!flush_rcache)
 			return 0;
 
 		/* Try replenishing IOVAs by flushing rcache. */
-		flushed_rcache = true;
+		flush_rcache = false;
 		for_each_online_cpu(cpu)
 			free_cpu_cached_iovas(cpu, iovad);
 		goto retry;