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Re: [Xen-devel] [RFC PATCH] page_alloc: use first half of higher order chunks when halving

On 03/25/14 06:20, Matt Wilson wrote:
On Tue, Mar 25, 2014 at 11:44:19AM +0000, Andrew Cooper wrote:
On 25/03/14 11:22, Matt Wilson wrote:
From: Matt Rushton <mrushton@xxxxxxxxxx>

This patch makes the Xen heap allocator use the first half of higher
order chunks instead of the second half when breaking them down for
smaller order allocations.

Linux currently remaps the memory overlapping PCI space one page at a
time. Before this change this resulted in the mfns being allocated in
reverse order and led to discontiguous dom0 memory. This forced dom0
to use bounce buffers for doing DMA and resulted in poor performance.

This change more gracefully handles the dom0 use case and returns
contiguous memory for subsequent allocations.

Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Matt Rushton <mrushton@xxxxxxxxxx>
Signed-off-by: Matt Wilson <msw@xxxxxxxxxx>
How does dom0 work out that it is safe to join multiple pfns into a dma
buffer without the swiotlb?
I'm not familiar enough with how this works to say. Perhaps Matt R.
can chime in during his day. My guess is that xen_swiotlb_alloc_coherent()
avoids allocating a contiguous region if the pages allocated already
happen to be physically contiguous.

Konrad, can you enlighten us? The setup code in question that does the
remapping one page at a time is in arch/x86/xen/setup.c.

The swiotlb code will check if the underlying mfns are contiguous and use a bounce buffer if and only if they are not. Everything goes through the swiotlb via the normal Linux dma apis it's just a matter of if it uses a bounce buffer or not.

  xen/common/page_alloc.c |    5 +++--
  1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 601319c..27e7f18 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -677,9 +677,10 @@ static struct page_info *alloc_heap_pages(
      /* We may have to halve the chunk a number of times. */
      while ( j != order )
-        PFN_ORDER(pg) = --j;
+        struct page_info *pg2;
At the very least, Xen style mandates a blank line after this variable

+        pg2 = pg + (1 << --j);
+        PFN_ORDER(pg) = j;
          page_list_add_tail(pg, &heap(node, zone, j));
-        pg += 1 << j;
ASSERT(avail[node][zone] >= request);

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