On Fri, 10 Jul 2015, Konrad Rzeszutek Wilk wrote:

On Thu, Jul 09, 2015 at 09:42:21PM +0100, Julien Grall wrote:

quoted

When Linux is using 64K page granularity, every page will be slipt in
multiple non-contiguous 4K MFN (page granularity of Xen).

But you don't care about that on the Linux layer I think?

As in, is there an SWIOTLB that does PFN to MFN and vice-versa
translation?

I thought that ARM guests are not exposed to the MFN<->PFN logic
and trying to figure that out to not screw up the DMA engine
on a PCIe device slurping up contingous MFNs which don't map
to contingous PFNs?

Dom0 is mapped 1:1, so pfn == mfn normally, however grant maps
unavoidably screw up the 1:1, so the swiotlb jumps in to save the day
when a foreign granted page is involved in a dma operation.

Regarding xen_biovec_phys_mergeable, we could check that all the pfn ==
mfn and return true in that case.

quoted

I'm not sure how to handle efficiently the check to know whether we can
merge 2 biovec with a such case. So for now, always says that biovec are
not mergeable.

Signed-off-by: Julien Grall <redacted>
Cc: Konrad Rzeszutek Wilk <redacted>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <redacted>
---
    Changes in v2:
        - Remove the workaround and check if the Linux page granularity
        is the same as Xen or not
---
 drivers/xen/biomerge.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
index 0edb91c..571567c 100644
--- a/drivers/xen/biomerge.c
+++ b/drivers/xen/biomerge.c

@@ -6,10 +6,17 @@
 bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
 			       const struct bio_vec *vec2)
 {
+#if XEN_PAGE_SIZE == PAGE_SIZE
 	unsigned long mfn1 = pfn_to_mfn(page_to_pfn(vec1->bv_page));
 	unsigned long mfn2 = pfn_to_mfn(page_to_pfn(vec2->bv_page));
 
 	return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) &&
 		((mfn1 == mfn2) || ((mfn1+1) == mfn2));
+#else
+	/* XXX: bio_vec are not mergeable when using different page size in
+	 * Xen and Linux
+	 */
+	return 0;
+#endif
 }
 EXPORT_SYMBOL(xen_biovec_phys_mergeable);

-- 
2.1.4