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


On 18/05/2015 13:54, Wei Liu wrote:
On Mon, May 18, 2015 at 01:11:26PM +0100, Julien Grall wrote:
On 15/05/15 16:31, Wei Liu wrote:
On Fri, May 15, 2015 at 01:35:42PM +0100, Julien Grall wrote:
On 15/05/15 03:35, Wei Liu wrote:
On Thu, May 14, 2015 at 06:01:01PM +0100, Julien Grall wrote:
The PV network protocol is using 4KB page granularity. The goal of this
patch is to allow a Linux using 64KB page granularity working as a
network backend on a non-modified Xen.

It's only necessary to adapt the ring size and break skb data in small
chunk of 4KB. The rest of the code is relying on the grant table code.

Although only simple workload is working (dhcp request, ping). If I try
to use wget in the guest, it will stall until a tcpdump is started on
the vif interface in DOM0. I wasn't able to find why.

I think in wget workload you're more likely to break down 64K pages to
4K pages. Some of your calculation of mfn, offset might be wrong.

If so, why tcpdump on the vif interface would make wget suddenly
working? Does it make netback use a different path?

No, but if might make core network component behave differently, this is
only my suspicion.

Do you see malformed packets with tcpdump?

I don't see any malformed packets with tcpdump. The connection is stalling
until tcpdump is started on the vif in dom0.

Hmm... Don't have immediate idea about this.

Ian said skb_orphan is called with tcpdump. If I remember correct that
would trigger the callback to release the slots in netback. It could be
that other part of Linux is holding onto the skbs for too long.

If you're wgetting from another host, I would suggest wgetting from Dom0
to limit the problem between Dom0 and DomU.

Thanks to Wei, I was able to narrow the problem. It looks like the problem is not coming from netback but somewhere else down in the network stack: wget/ssh between Dom0 64KB and DomU is working fine.

Although, wget/ssh between a guest and an external host doesn't work when Dom0 is using 64KB page granularity unless if I start a tcpdump on the vif in DOM0. Anyone an idea?

I have no issue to wget/ssh in DOM0 to an external host and the same kernel with 4KB page granularity (i.e same source code but rebuilt with 4KB) doesn't show any issue with wget/ssh in the guest.

This has been tested on AMD Seattle, the guest kernel is the same on every test (4KB page granularity).

I'm planning to give a try tomorrow on X-gene (ARM64 board and I think 64KB page granularity is supported) to see if I can reproduce the bug.

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.

  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

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.

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

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