[libvirt] error: server response too large

[Claudio Bley]

At Tue, 1 Oct 2013 17:19:56 +0100,
Daniel P. Berrange wrote:
> 
> On Tue, Oct 01, 2013 at 06:13:10PM +0200, Viktor Mihajlovski wrote:
> > On 10/01/2013 03:02 PM, Daniel P. Berrange wrote:
> > >On Mon, Sep 30, 2013 at 04:24:49PM +0100, Daniel P. Berrange wrote:
> > >>On Mon, Sep 30, 2013 at 05:23:33PM +0200, Michal Privoznik wrote:
> > >>>On 30.09.2013 17:02, Claudio Bley wrote:
> > >>>>Hi.
> > >>>>
> > >>>>When trying to do a screenshot of a remote domain connected via
> > >>>>qemu+tcp (for testing purposes only), I receive this error:
> > >>>>
> > >>>>----------------------------------------------------------------------
> > >>>>virsh -c qemu+tcp://dev/system
> > >>>>Welcome to virsh, the virtualization interactive terminal.
> > >>>>
> > >>>>Type:  'help' for help with commands
> > >>>>        'quit' to quit
> > >>>>
> > >>>>virsh # version
> > >>>>Compiled against library: libvir 0.9.8
> > >>>>Using library: libvir 0.9.8
> > >>>>Using API: QEMU 0.9.8
> > >>>>Running hypervisor: QEMU 1.5.1
> > >>>>
> > >>>>virsh # screenshot 2 /tmp/screendump
> > >>>>error: could not receive data from domain 2
> > >>>>error: packet 1048600 bytes received from server too large, want 262144
> > >>>>
> > >>>>virsh # 2013-09-30 14:47:05.158+0000: 21646: info : libvirt version: 0.9.8
> > >>>>2013-09-30 14:47:05.158+0000: 21646: warning : virNetClientIncomingEvent:1660 : Something went wrong during async message processing
> > >>>>----------------------------------------------------------------------
> > >>>>
> > >>>>I'm using Ubuntu LTS 12.04.3, latest version in the repo which is
> > >>>>0.9.8-2ubuntu17.13.
> > >>>
> > >This problem was fixed in
> > >
> > >commit 27e81517a876dcb738dd8a9bb2e0e68d71c3b7e3
> > >Author: Daniel P. Berrange <berrange at redhat.com>
> > >Date:   Mon Sep 30 17:27:51 2013 +0100
> > >
> > >     Fix max stream packet size for old clients
> > >
> > >     The libvirtd server pushes data out to clients. It does not
> > >     know what protocol version the client might have, so must be
> > >     conservative and use the old payload limits. ie send no more
> > >     than 256kb of data per packet.
> > >
> > >
> > >And backported to stable branches.
> > >
> > I tried libvirt 1.1.3 containing the commit on the server and
> > Ubuntu libvirt 0.9.8 as the client and still have the issue
> > if the message is greater than 256KB. The problem is that
> > the old client checks the message size during decode (which is
> > greater than the client's max message size).
> > 
> > Not sure how to get out of this ...
> 
> What were you doing to get a message greater than 256KB ? This
> patch did not attempt to fix all possible combinations. If a
> API call such as virConnectListAllDomains has so much data that
> it requires > 256KB to encode, this fix won't solve that. There
> is nothing we can do for API calls which have a genuine need to
> exceed the old size limit.
> 
> I was only addressing the case of virStreamPtr data which was
> mistakenly hardcoded in the server to try sending 16 MB of data
> at once. I switched it to only try to send 256 KB of data per
> stream packet.

I've also tested your patch, and it seems 256 KB of data is still a
bit too large:

virsh # screenshot 2 /tmp/test
error: could not receive data from domain 2
error: packet 262168 bytes received from server too large, want 262144

The max payload size is computed as:

VIR_NET_MESSAGE_MAX = 16777216
VIR_NET_MESSAGE_HEADER_MAX = 24
VIR_NET_MESSAGE_PAYLOAD_MAX = (VIR_NET_MESSAGE_MAX - VIR_NET_MESSAGE_HEADER_MAX) = 16777192

So, it seems the legacy max payload size is actually 262120; I tested
it and it works.

------------------- 8< ------ >8 ---------------------------
Subject: [PATCH] Adjust legacy max payload size to account for header
 information
Organization: AV-Test GmbH, Germany

Commit 27e81517a87 set the payload size to 256 KB, which is
actually the max packet size, including the size of the header.

Reduce this by VIR_NET_MESSAGE_HEADER_MAX (24) and set
VIR_NET_MESSAGE_LEGACY_PAYLOAD_MAX to 262120.

Signed-off-by: Claudio Bley <cbley at av-test.de>
---
 src/rpc/virnetprotocol.x |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/rpc/virnetprotocol.x b/src/rpc/virnetprotocol.x
index 1eae7cb..7b6f753 100644
--- a/src/rpc/virnetprotocol.x
+++ b/src/rpc/virnetprotocol.x
@@ -55,7 +55,7 @@ const VIR_NET_MESSAGE_INITIAL = 65536;
  * payload size. We need to remember this for compat with
  * old clients.
  */
-const VIR_NET_MESSAGE_LEGACY_PAYLOAD_MAX = 262144;
+const VIR_NET_MESSAGE_LEGACY_PAYLOAD_MAX = 262120;

 /* Maximum total message size (serialised). */
 const VIR_NET_MESSAGE_MAX = 16777216;
------------------- 8< ------ >8 ---------------------------
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