Re: [Xen-devel] memory hotplug for domUs

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Jan 20, 2017 at 11:08:54AM +0000, Andrew Cooper wrote:
> On 20/01/17 09:26, Juergen Gross wrote:

Cc-ing Daniel as he wrote the Xen Balloon memory hotplug code.

> > Recently Jim asked me why he can use "xl mem-max" to raise the allowed
> > memory size of a domain in the hypervisor above the configured maxmem
> > limit of the domain, but not use "xl mem-set" to balloon the domain up
> > to this value later.
> >
> > I thought libxl_domain_setmaxmem() being buggy as it doesn't modify the
> > memory/static-max value in Xenstore and posted a patch adding the
> > Xenstore modification.
> >
> > Later I had an IRC discussion with Ian on #xendevel with the following
> > outcome:
> >
> > xl mem-max is defined to do things as it does without my patch. OTOH
> > this is documented very poorly. I'll send a patch enhancing the xl man
> > page in this regard.
> >
> > For support of memory hotplug in a domain (adding memory above the
> > current defined maximum memory as in Xenstore memory/static-max) we
> > decided to add a new option to xl.
> >
> > Basically doing such a kind of memory hotplug requires the capability
> > of the guest to support this feature. PV Linux kernel has support for
> > it if the kernel has been built with CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
> > set. For HVM guests support is more difficult as there has to be some
> > reserved space in the memory map of the guest for hotplugged memory.
> >
> > Ian suggested to let the guest write a Xenstore entry indicating
> > support of memory hotplug in order for Xen tools to decide whether such
> > an operation should be supported. As this entry isn't being written by
> > current kernels which do support hotplug there has to be some kind of
> > "force" flag to override the test for presence of the Xenstore entry.
> >
> > We first thought to enhance "xl mem-set", but after some more thinking
> > about it I'd rather add a new xl command, e.g. "mem-add" (we could later
> > even add "mem-remove" to support memory unplug).
> >
> > xl mem-add would add the specified amount of memory to a running domain
> > and it would raise memory/static-max accordingly. The first version
> > would support PV domains only, but future support for HVM could be
> > possible (the domain configuration would have to be enhanced to create
> > a memory map entry indicating hot-pluggable memory). Other future
> > enhancements would include the possibility to add memory to a vnuma
> > node of the domain, possibly taking the memory from a specified numa
> > node of the host.
> >
> > In case this proposal is accepted I can write patches (libxl and Linux
> > kernel).
> 
> For PV guests, this sounds plausible.
> 
> For HVM/PVH guests however, ACPI Memory Hotplug.  That way it will work
> for all operating systems, not just Linux.  (Lets please not waste time
> building a somewhat-square wheel which only fits on a Linux shaped axle :) )
> 
> ~Andrew
> 
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