[Xen-devel] Re: [PATCHEs]: support more than 32 VCPUs in guests

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Wed, 09 Jun 2010 17:49:52 -0700
Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:

> On 06/09/2010 05:08 PM, Mukesh Rathor wrote:
> >> Why BUG_ON if the number of cpus is too high?  Why not just ignore
> >> the excess ones?
> >>     
> > Yeah, that was my first thought also... but then i realized i
> > couldn't just ignore the excess cpus in that function, but would
> > need to go back and fixup all the cpu_present, cpu_online, etc maps
> > (and any assoc data structs, if any), and it just didn't seem worth
> > it in the 2.6.18* kernels at least. Would have been easier to do if
> > the vcpu setup function returned a value instead of being void. 
> >   
> 
> Yes, but if have_vcpu_info_placement ends up being false (which is
> tested before any other cpus are brought up) then you can simply fail
> to online the ones above the limit.
> 
> BUG_ON is way too brutal.  You need to fail more softly.
> 
>     J

Well, BUG_ON is only triggered if booting more than 32 VCPUs on a *very
old* xen (pre xen 3.1.0). 

Looking at code closely, we could just set setup_max_cpus to 32 some
where in xen function, perhaps even in xen_vcpu_setup(). That way
later in smp_init() it would just be ok. 
One thing tho, the per cpus areas are already setup at that point, so that 
would need to be cleaned. BTW, I don't understand why 
have_vcpu_info_placement is set to 0 in xen_guest_init()? 

What minimum version of xen is required to run pvops kernel?

thanks,
Mukesh


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