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[PATCH v2] virtio_net: fix PAGE_SIZE > 64k

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I don't have any guests with PAGE_SIZE > 64k but the
code seems to be clearly broken in that case
as PAGE_SIZE / MERGEABLE_BUFFER_ALIGN will need
more than 8 bit and so the code in mergeable_ctx_to_buf_address
does not give us the actual true size.

Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

changes from v1:
	fix build warnings

 drivers/net/virtio_net.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 4a10500..4dc373b 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -48,8 +48,16 @@ module_param(gso, bool, 0444);
  */
 DECLARE_EWMA(pkt_len, 1, 64)
 
+/* With mergeable buffers we align buffer address and use the low bits to
+ * encode its true size. Buffer size is up to 1 page so we need to align to
+ * square root of page size to ensure we reserve enough bits to encode the true
+ * size.
+ */
+#define MERGEABLE_BUFFER_MIN_ALIGN_SHIFT ((PAGE_SHIFT + 1) / 2)
+
 /* Minimum alignment for mergeable packet buffers. */
-#define MERGEABLE_BUFFER_ALIGN max(L1_CACHE_BYTES, 256)
+#define MERGEABLE_BUFFER_ALIGN max(L1_CACHE_BYTES, \
+				   1 << MERGEABLE_BUFFER_MIN_ALIGN_SHIFT)
 
 #define VIRTNET_DRIVER_VERSION "1.0.0"
 
-- 
MST

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[PATCH v2] virtio_net: fix PAGE_SIZE > 64k "Michael S. Tsirkin" <mst@redhat.com> - 2017-01-23 20:40 +0100
  Re: [PATCH v2] virtio_net: fix PAGE_SIZE > 64k David Miller <davem@davemloft.net> - 2017-01-24 20:50 +0100
    Re: [PATCH v2] virtio_net: fix PAGE_SIZE > 64k "Michael S. Tsirkin" <mst@redhat.com> - 2017-01-24 21:00 +0100
      Re: [PATCH v2] virtio_net: fix PAGE_SIZE > 64k David Miller <davem@davemloft.net> - 2017-01-24 21:20 +0100
        Re: [PATCH v2] virtio_net: fix PAGE_SIZE > 64k "Michael S. Tsirkin" <mst@redhat.com> - 2017-01-24 21:50 +0100
          Re: [PATCH v2] virtio_net: fix PAGE_SIZE > 64k David Miller <davem@davemloft.net> - 2017-01-24 22:00 +0100
            Re: [PATCH v2] virtio_net: fix PAGE_SIZE > 64k "Michael S. Tsirkin" <mst@redhat.com> - 2017-01-24 22:10 +0100
              Re: [PATCH v2] virtio_net: fix PAGE_SIZE > 64k David Miller <davem@davemloft.net> - 2017-01-24 22:20 +0100
                Re: [PATCH v2] virtio_net: fix PAGE_SIZE > 64k "Michael S. Tsirkin" <mst@redhat.com> - 2017-01-24 23:00 +0100
                Re: [PATCH v2] virtio_net: fix PAGE_SIZE > 64k John Fastabend <john.fastabend@gmail.com> - 2017-01-25 04:50 +0100

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