Re: [Xen-devel] Claim mode and HVM PoD interact badly

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Jan 10, 2014 at 10:41:05AM -0500, Konrad Rzeszutek Wilk wrote:
> On Fri, Jan 10, 2014 at 03:10:48PM +0000, Wei Liu wrote:
> > On Fri, Jan 10, 2014 at 09:58:07AM -0500, Konrad Rzeszutek Wilk wrote:
> > > On Fri, Jan 10, 2014 at 11:59:42AM +0000, Ian Campbell wrote:
> > > > create ^
> > > > owner Wei Liu <wei.liu2@xxxxxxxxxx>
> > > > 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.
> > > 
> > 
> > Huh? My debug message says otherwise. It tried to claim 248MB (256MB -
> > 8MB video ram). Did I misread your message...
> 
> The 'claim' being the hypercall to set the 'clamp' on how much memory
> the guest can allocate. This is based on:
> 
> 242     unsigned long i, nr_pages = args->mem_size >> PAGE_SHIFT;

This is in fact initialized to 'maxmem' in guest's config file and

243     unsigned long target_pages = args->mem_target >> PAGE_SHIFT;

This is in fact 'memory' in guest's config file.

So when you try to claim "maxmem" and the current limit is "memory" it
would not work.

So guest should only claim target_pages sans 0x20 pages if PoD enabled.
Oh this is what your initial patch did. I don't know whether this is
conceptually correct though. :-P

Further more, should guest only allow to claim target_pages, regardless
whether PoD is enabled? When only "memory" is specify, "maxmem"
equals to "memory". So conceptually what we really care about is
"memory" not "maxmem".

Wei.


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