[PATCH V2] xen-swiotlb: exchange memory with Xen only when pages are contiguous

[Joe Jin]

Commit 4855c92dbb7 "xen-swiotlb: fix the check condition for
xen_swiotlb_free_coherent" only fixed memory address check condition
on xen_swiotlb_free_coherent(), when memory was not physically
contiguous and tried to exchanged with Xen via
xen_destroy_contiguous_region which lead to kernel panic.

The correct check conditions to make Xen hypercall to revert the
memory back from its 32-bit pool are:
1) Above its DMA bit mask (for example 32-bit devices can only address
up to 4GB, and we may want 4GB+2K), and
2) It's physically contiguous.

Thank you Boris for pointing it out.

Fixes: 4855c92dbb7 ("xen-swiotlb: fix the check condition for
xen_swiotlb_free_coherent"

Signed-off-by: Joe Jin <joe.jin@xxxxxxxxxx>
Reported-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Christoph Helwig <hch@xxxxxx>
Cc: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
Cc: John Sobecki <john.sobecki@xxxxxxxxxx>
---
 drivers/xen/swiotlb-xen.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
index f5c1af4ce9ab..aed92fa019f9 100644
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c
@@ -357,8 +357,8 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t size, void *vaddr,
 	/* Convert the size to actually allocated. */
 	size = 1UL << (order + XEN_PAGE_SHIFT);
 
-	if (((dev_addr + size - 1 <= dma_mask)) ||
-	    range_straddles_page_boundary(phys, size))
+	if ((dev_addr + size - 1 <= dma_mask) &&
+	    !range_straddles_page_boundary(phys, size))
 		xen_destroy_contiguous_region(phys, order);
 
 	xen_free_coherent_pages(hwdev, size, vaddr, (dma_addr_t)phys, attrs);
-- 
2.17.1 (Apple Git-112)