On Wed, Oct 23, 2013 at 06:34:13PM +0100, Stefano Stabellini wrote:

quoted

On Thu, Oct 17, 2013 at 06:43:33PM +0100, Stefano Stabellini wrote:

quoted

xen_unmap_single calls xen_bus_to_phys, then passes dev_addr to
is_xen_swiotlb_buffer that calls xen_bus_to_phys again.

It does?

I see it call mfn_to_local_pfn which calls pfn_to_mfn and
get_phys_to_machine (which I presume is going to be a slow
operation on ARM).

Yeah, I am trying to reduce the number of pfn_to_mfn and mfn_to_pfn
translations.
That said, this patch is not needed to have a fully functional
swiotlb-xen on arm and I would OK with postponing it.

OK, lets do that as it requires some hard thinking.

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xen_bus_to_phys is potentially a slow operation that might involve
walking a tree.
We can avoid calling xen_bus_to_phys twice by removing the
is_xen_swiotlb_buffer call and doing the check directly in
xen_unmap_single using the physical address.

Signed-off-by: Stefano Stabellini <redacted>

Have you tested this with a PFN that is in a foreign domain?

That would be blkback. I only tested blkback on ARM, I'll test on x86
too.

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I am bit uncomfortable with this - I recall when developing this
that I had hit some odd cases of doing DMA of a page to a guest
and it not always working. It was the case of not detecting whether
the pfn was foreign or not. Sadly I can't recall the details.

I would think that this problem would also be present with ARM?
But perhaps all the new changes in the pfn_to_mfn that had lately
been done have fixed this?

No, because on ARM the guest is autotranslate so the pfn returned by
mfn_to_pfn is always "local".
However thanks to the m2p_override now we should get the "local" pfn
from mfn_to_pfn on x86 too.
I'll test this case though.

Right, so maybe the pfn_to_mfn_local test is useless nowadays?

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---
 drivers/xen/swiotlb-xen.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
index 4d50058..6d9ca61 100644
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c

@@ -426,8 +426,8 @@ static void xen_unmap_single(struct device *hwdev, dma_addr_t dev_addr,
 
 	xen_dma_unmap_page(hwdev, paddr, size, dir, attrs);
 
-	/* NOTE: We use dev_addr here, not paddr! */
-	if (is_xen_swiotlb_buffer(dev_addr)) {
+	if (paddr >= virt_to_phys(xen_io_tlb_start) &&
+		paddr < virt_to_phys(xen_io_tlb_end)) {
 		swiotlb_tbl_unmap_single(hwdev, paddr, size, dir);
 		return;
 	}

-- 
1.7.2.5