Re: [Qemu-devel] [Bug 1585533] [NEW] cache-miss-rate / Invalid JSON

[Eric Blake <eblake@redhat.com>]

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On 05/25/2016 02:46 AM, Marc Brothier wrote:
> Public bug reported:
> 
> Hi,
> 
> We have VMs which were started with an older version than qemu 2.1 which
> added "cache-miss-rate" property for XBZRLECacheStats. While trying to
> migrate the VM to a new host which is running a higher version (2.3) of
> Qemu we got an exception:
> 
> virJSONValueFromString:1642 : internal error: cannot parse json {"return": {"expected-downtime": 1, "xbzrle-cache": {"bytes": 0, "cache-size": 67108864, "cache-miss-rate": -nan, "pages": 0, "overflow": 0, "cache-miss": 8933}, "status": "active", "disk": {"total": 429496729600, "dirty-sync-count": 0, "remaining": 193896382464, "mbps": 0, "transferred": 235600347136, "duplicate": 0, "dirty-pages-rate": 0, "skipped": 0, "normal-bytes": 0, "normal": 0}, "setup-time": 13, "total-time": 1543124, "ram": {"total": 8599183360, "dirty-sync-count": 4, "remaining": 30695424, "mbps": 830.636997, "transferred": 3100448901, "duplicate": 1358341, "dirty-pages-rate": 7, "skipped": 0, "normal-bytes": 3082199040, "normal": 752490}}, "id": "libvirt-186200"}: lexical error: malformed number, a digit is required after the minus sign.
>           67108864, "cache-miss-rate": -nan, "pages": 0, "overflow": 0
>                      (right here) ------^
> 
> virNetClientStreamRaiseError:191 : stream aborted at client request

Wow - I've known we have a problem with qemu emitting non-compliant
JSON, but this proves that it is fatal to libvirt. I guess my series on
improving the JSON parser [1] should consider doing a fallback to
s/NaN/0/ and s/Inf/DBL_MAX/ rather than completely erroring out when a
client tries to request it. Meanwhile, it's an easy patch to qemu to
avoid division by zero when generating cache-miss-rate.

[1] https://lists.gnu.org/archive/html/qemu-devel/2016-05/msg03424.html

> 
> 
> Would it be possible to improve the JSON parser to skip the key if the value is incorrect

Libvirt uses libyajl to parse JSON, and libyajl has an outstanding bug
request to support extensions to JSON such as parsing non-finite floats.
 Since there has been no upstream reaction to the bug request, I
seriously doubt it will happen any time soon, so any change to tolerate
NaN in libvirt would have to be a one-off patch.  It sounds like the
better fix is to make qemu emit valid JSON in the first place, rather
than making libvirt deal with broken JSON from qemu.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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