Bug - Xen 4.3 - xl ignores maxmem setting in domU config file

[James Dingwall <james-xen@dingwall.me.uk>]

Hi,

I am having some memory ballooning problems which only seem to have 
appeared since upgrading to Xen 4.3.  In short if I set the domU 
configuration file as:

memory = 512
maxmem = 1024

Then even under memory pressure the guest domain does not balloon past 
the value of the 'memory' parameter.

xl info shows plenty of available memory in dom0:
total_memory           : 32767
free_memory            : 12445


xl list -l on the domain shows:
     "max_memkb": 1048576,
     "target_memkb": 524288,

xl top shows:
            NAME        guestdom
           STATE        --b---
        CPU(sec)        79
          CPU(%)        1.2
          MEM(k)        343116
          MEM(%)        1.0
       MAXMEM(k)        525312
       MAXMEM(%)        1.6
           VCPUS        2
            NETS        0
        NETTX(k)        0
        NETRX(k)        0
            VBDS        2
          VBD_OO        0
          VBD_RD        8639
          VBD_WR        129666
       VBD_RSECT        529846
       VBD_WSECT        1622593
            SSID        0


In the domU dmesg it seems as though the guest has some idea that it has 
a 'maxmem' size of 1024Mb:

[    0.000000] Memory: 408932K/1048188K available (4562K kernel code, 
541K rwdata, 1740K rodata, 924K init, 644K bss, 639256K reserved)


but /proc/iomap seems to have reduced it to 'memory':
00000000-00000fff : reserved
00001000-0009ffff : System RAM
000a0000-000fffff : reserved
  000f0000-000fffff : System ROM
00100000-1fffffff : System RAM
  01000000-01474c38 : Kernel code
  01474c39-016b167f : Kernel data
  017a0000-01840fff : Kernel bss
20000000-d7feffff : Unusable memory
d7ff0000-d7ffdfff : ACPI Tables
d7ffe000-d7ffffff : ACPI Non-volatile Storage
100000000-11fffffff : System RAM


After running 
xl mem-max guestdom 1024

xl list -l remains
   "max_memkb": 1048576,
   "target_memkb": 524288,

xl top has changed:
            NAME        guestdom
           STATE        --b---
        CPU(sec)        98
          CPU(%)        0.9
          MEM(k)        343116
          MEM(%)        1.0
       MAXMEM(k)        1049600
       MAXMEM(%)        3.1
           VCPUS        2
            NETS        0
        NETTX(k)        0
        NETRX(k)        0
            VBDS        2
          VBD_OO        0
          VBD_RD        9068
          VBD_WR        130257
       VBD_RSECT        549640
       VBD_WSECT        1628666
            SSID        0


Although /proc/iomem doesn't change the guest has no problem ballooning 
up to the 'maxmem' value.  It seems that this is a bug with xl not 
setting the appropriate paramter somewhere when the domain is created.

The domU and dom0 kernel version is 3.11.6, Xen is 4.3.  Please let me 
know if examinging any other settings before/after the xl mem-max 
command is run would be helpful.

Thanks,
James