Messages in this thread Patch in this message | | | From | Wei Yang <> | Subject | [RFC PATCH] iommu: enable the last bit in iommu_area_alloc() | Date | Sun, 20 Sep 2015 10:14:56 +0800 |
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In 'commit a66022c45775 ("iommu-helper: use bitmap library")', iommu_area_alloc() uses bitmap_find_next_zero_area() to lookup available iommu space.
When given "start, size, nr", bitmap_find_next_zero_area() is looking for a range with nr zero bit in [start, size) instead of [start, size]. This means the last bit is already excluded. By decrease size at the beginning, the last iommu page will not be allocated.
This patch removes the decrease on size.
Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
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I may missed something, while the code makes me a little confused.
I found two users of iommu_area_alloc(), one in powernv platform and one in lib/iommu-common.c. The "limit", which is passed to bitmap_find_next_zero_area() as the "size" are both set to pool->end in these two cases. While the pool->end in these two cases are calculated differently.
On powernv platform, iommu_init_table() sets p->end = p->start + tbl->poolsize; While in iommu_tbl_pool_init(), pools[i].end = pools[i].start + iommu->poolsize -1;
In both case, it will not do harm to system, except we have one more less iommu page in the second case.
And then in current code, iommu_area_alloc() will decrease the size by one again.
I may missed something, currently I think the implementation in powernv platform is correct while iommu_area_alloc() eats the last iommu page.
Tests:
I have apply this on top of v4.2 and enforce to use dma_iommu_ops for each device. Transfered on guest image with 4GB by scp and check the checksum are the same.
--- lib/iommu-helper.c | 2 -- 1 file changed, 2 deletions(-)
diff --git a/lib/iommu-helper.c b/lib/iommu-helper.c index c27e269..2866004 100644 --- a/lib/iommu-helper.c +++ b/lib/iommu-helper.c @@ -23,8 +23,6 @@ unsigned long iommu_area_alloc(unsigned long *map, unsigned long size, { unsigned long index; - /* We don't want the last of the limit */ - size -= 1; again: index = bitmap_find_next_zero_area(map, size, start, nr, align_mask); if (index < size) { -- 2.5.0
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