[Xen-devel] Re: Memory-hotplug support for x86_64 domUs?

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Radhakrishnan, Prashanth wrote:
> Thanks for the info!
>
>   
> > But it isn't very useful at present.
> >     
> Can you please elaborate why. Is it not possible to add memory to VMs
> without rebooting them?
>   
Sorry,  I was unclear.  It's definitely possible in principle to add 
memory to VMs on the fly, but it just isn't implemented in the pvops 
kernels at the moment.  The patch I posted (that you referred to) makes 
the balloon driver call into the memory hotplug subsystem to add new 
memory to the system.  But the new memory is not populated all at once; 
it's just a set of unbacked pageframes to start with.  The memory 
hotplug system automatically puts a file in /sys referring to the newly 
added "memory", and allows you to "online" it all.  Unfortunately, since 
there's no real memory behind the pages, this will almost immediately 
cause your domain to crash.
I'd like to make it so that the balloon driver can control onlining 
pages on a page-by-page basis, and there should be no usermode interface 
for this memory.  But so far we haven't worked out mutually acceptible 
way to do this.
I could post the straightforward balloon/hotplug patches, but I'm not 
comfortable doing so when they leave such an dangerous pit for people to 
fall into.
   J

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