Re: Claim mode and HVM PoD interact badly

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Fri, Jan 10, 2014 at 11:59:42AM +0000, Ian Campbell wrote:
> create ^
> owner Wei Liu <wei.liu2@citrix.com>
> thanks
> 
> On Fri, 2014-01-10 at 11:56 +0000, Wei Liu wrote:
> > When I have following configuration in HVM config file:
> >   memory=128
> >   maxmem=256
> > and have claim_mode=1 in /etc/xen/xl.conf, xl create fails with
> > 
> > xc: error: Could not allocate memory for HVM guest as we cannot claim memory! (22 = Invalid argument): Internal error
> > libxl: error: libxl_dom.c:647:libxl__build_hvm: hvm building failed
> > libxl: error: libxl_create.c:1000:domcreate_rebuild_done: cannot (re-)build domain: -3
> > libxl: error: libxl_dm.c:1467:kill_device_model: unable to find device model pid in /local/domain/82/image/device-model-pid
> > libxl: error: libxl.c:1425:libxl__destroy_domid: libxl__destroy_device_model failed for 82
> > 
> > With claim_mode=0, I can sucessfuly create HVM guest.
> 
> Is it trying to claim 256M instead of 128M? (although the likelyhood

No. 128MB actually.

> that you only have 128-255M free is quite low, or are you
> autoballooning?)

This patch fixes it for me. It basically sets the amount of pages
claimed to be 'maxmem' instead of 'memory' for PoD.

I don't know PoD very well, and this claim is only valid during the
allocation of the guests memory - so the 'target_pages' value might be
the wrong one. However looking at the hypervisor's
'p2m_pod_set_mem_target' I see this comment:

 316  *     B <T': Set the PoD cache size equal to the number of outstanding PoD
 317  *   entries.  The balloon driver will deflate the balloon to give back
 318  *   the remainder of the ram to the guest OS.

Which implies to me that we _need_ the 'maxmem' amount of memory at boot time.
And then it is the responsibility of the balloon driver to give the memory
back (and this is where the 'static-max' et al come in play to tell the
balloon driver to balloon out).


diff --git a/tools/libxc/xc_hvm_build_x86.c b/tools/libxc/xc_hvm_build_x86.c
index 77bd365..65e9577 100644
--- a/tools/libxc/xc_hvm_build_x86.c
+++ b/tools/libxc/xc_hvm_build_x86.c
@@ -335,7 +335,12 @@ static int setup_guest(xc_interface *xch,
 
     /* try to claim pages for early warning of insufficient memory available */
     if ( claim_enabled ) {
-        rc = xc_domain_claim_pages(xch, dom, nr_pages - cur_pages);
+        unsigned long nr = nr_pages - cur_pages;
+
+        if ( pod_mode )
+            nr = target_pages - 0x20;
+
+        rc = xc_domain_claim_pages(xch, dom, nr);
         if ( rc != 0 )
         {
             PERROR("Could not allocate memory for HVM guest as we cannot claim memory!");