[Xen-devel] [RFC 21/23] net/xen-netback: Make it running on 64KB page granularity

On Tue, May 19, 2015 at 11:56:39PM +0100, Julien Grall wrote:

quoted

quoted

diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
index 0eda6e9..c2a5402 100644
--- a/drivers/net/xen-netback/common.h
+++ b/drivers/net/xen-netback/common.h

@@ -204,7 +204,7 @@ struct xenvif_queue { /* Per-queue data for xenvif */
 /* Maximum number of Rx slots a to-guest packet may use, including the
  * slot needed for GSO meta-data.
  */
-#define XEN_NETBK_RX_SLOTS_MAX (MAX_SKB_FRAGS + 1)
+#define XEN_NETBK_RX_SLOTS_MAX ((MAX_SKB_FRAGS + 1) * XEN_PFN_PER_PAGE)

 enum state_bit_shift {
        /* This bit marks that the vif is connected */

The function xenvif_wait_for_rx_work never returns. I guess it's because there
is not enough slot available.

For 64KB page granularity we ask for 16 times more slots than 4KB page
granularity. Although, it's very unlikely that all the slot will be used.

FWIW I pointed out the same problem on blkfront.

This is not going to work. The ring in netfront / netback has only 256
slots. Now you ask for netback to reserve more than 256 slots -- (17 +
1) * (64 / 4) = 288, which can never be fulfilled. See the call to
xenvif_rx_ring_slots_available.

I think XEN_NETBK_RX_SLOTS_MAX derived from the fact the each packet to
the guest cannot be larger than 64K. So you might be able to

#define XEN_NETBK_RX_SLOTS_MAX ((65536 / XEN_PAGE_SIZE) + 1)

I didn't know that packet cannot be larger than 64KB. That's simply a lot
the problem.

I think about this more, you will need one more slot for GSO
information, so make it ((65536 / XEN_PAGE_SIZE) + 1 + 1).

quoted

Blk driver may have a different story. But the default ring size (1
page) yields even less slots than net (given that sizeof(union(req/rsp))
is larger IIRC).

I will see with Roger for Blkback.


-- 
Julien Grall