Re: [Xen-devel] [PATCH v3 1/3] libxl: xl mem-max et consortes must update static-max in xenstore too [and 1 more messages]

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Daniel Kiper writes ("Re: [Xen-devel] [PATCH v3 1/3] libxl: xl mem-max et 
consortes must update static-max in xenstore too [and 1 more messages]"):
> I think that we should not care (from a host point of view) which
> mechanism is used in a guest to increase its memory size. From the
> host point of view in balloon case and in memory hotplug case we
> just allocate pages for the guest.  We do not care where they will
> be placed and when they will be used. That is it.

Yes, exactly.

> However, we should create a mechanism which allows the guest to
> allocate memory above limit set at boot and not eat all host memory
> in uncontrolled manner (e.g. using memory hotplug mechanism). Last
> is provided by xl now but former is not.

Yes.  That limit mechanism is the Xen memory limit, which by default
is set by xl mem-set and with your changes can be independently set
with xl mem-max.

> Currently there is no way to increase memory limit above memory
> limit set at boot (maxmem).

That's the wrong way to think about it.  Currently xl will allow
neither the target nor the xen maximum to be increased beyond the boot
maximum, because they think that won't work.

> Now I think we should just decide which value should be
> used as a cap reference: static-max or "xen maximum". Both are not
> perfect and some recalculation is always needed to establish real
> cap internaly in the guest.

I don't know what "cap reference" is.

It seems to me that if the guest could in principle do memory hotplug
without assistance from the toolstack, all that's needed is:

 * Guest advertises memory hotplug capability in xenstore
   (presumably it advertises a maximum figure, rather than
    just promising to be able to use any amount of memory, since
    most guests will be able to hotplug only a certain amount?)

 * When memory hotplug limit is advertised, libxl ignores static-max
   in xenstore in favour of the guest's advertised capability.
   xl mem-set (and if different, xl mem-max) can be told to set the
   guest's memory target (in xenstore), and the xen memory limit,
   to more than the boot time memory.  When this happens the guest's
   balloon/hotplug driver can hotplug memory appropriately to
   bring the guest's memory usage to the requested target.

Ian.

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