Re: [PATCH for-4.5 v3] Xen and tools: Fix listing of vcpus when domains lacking any vcpus exist

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2014-10-21 at 17:46 +0100, Andrew Cooper wrote:
> On a system which looks like this:
> 
> [root@st04 ~]# xl list
> Name                                        ID   Mem VCPUs      State   Time(s)
> Domain-0                                     0   752     4     r-----   46699.3
> (null)                                       1     0     0     --p---       0.0
> (null)                                       2     0     0     --p---       0.0
> (null)                                       3     0     0     --p---       0.0
> badger                                      25     0     1     --p---       0.0
> 
> `xl vcpu-list` failes as so:
> 
> [root@st04 ~]# xl vcpu-list
> Name                                ID  VCPU   CPU State   Time(s) CPU Affinity
> Domain-0                             0     0    0   -b-   12171.0  all
> Domain-0                             0     1    1   -b-   11779.6  all
> Domain-0                             0     2    2   -b-   11599.0  all
> Domain-0                             0     3    3   r--   11007.0  all
> libxl: critical: libxl__calloc: libxl: FATAL ERROR: memory allocation failure (libxl__calloc, 4294935299 x 40)
> : Cannot allocate memory
> libxl: FATAL ERROR: memory allocation failure (libxl__calloc, 4294935299 x 40)
> 
> The root cause of this is in Xen.  getdomaininfo() has no way of expressing
> "this domain has no vcpus".  Previously, info->max_vcpu_id would be returned
> uninitialised in such a case.
> 
> Unfortunately, setting it to 0 as a default is not appropriate.  A max_vcpu_id
> of 0 and nr_online_cpus of 0 is the valid state for a single vcpu domain which
> is in the process of being destroyed.
> 
> As all components are required to add 1 to max_vcpu_id to get the number of
> vcpus, an id of ~0U is not valid to be used.  Explicitly define this as an
> invalid max vcpu value, and use it to express "no vcpus" in getdomaininfo()
> 
> In libxl, the issue is seen as libxl_list_vcpu() attempts to use the
> uninitialised domaininfo.max_vcpu_id for memory allocation.
> 
> Check domaininfo.max_vcpu_id against the new sentinel value
> XEN_INVALID_MAX_VCPU_ID, and return early.  This means that it is now valid
> for libxl_list_vcpu() to return NULL for a domain which lacks any vcpus.
> 
> As part of this change, remove the pointless call to libxl_get_max_cpus(),
> whose returned value is unconditionally clobbered in the for() loop.
> 
> Reported-by: Euan Harris <euan.harris@citrix.com>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Acked-by: Jan Beulich <jbeulich@suse.com>
> CC: Keir Fraser <keir@xen.org>

Tools-side:

Acked-by: Ian Campbell <Ian.Campbell@citrix.com>

(libxl_list_vcpu has some rather icky unconventional error handling.
sigh, not your problem here though)

Ian.