Re: [Qemu-devel] [RFC PATCH 0/9] ACPI memory hotplug

[Gleb Natapov]

On Mon, Apr 23, 2012 at 04:31:04PM +0300, Avi Kivity wrote:
> On 04/22/2012 05:20 PM, Gleb Natapov wrote:
> > On Sun, Apr 22, 2012 at 05:13:27PM +0300, Avi Kivity wrote:
> > > On 04/22/2012 05:09 PM, Gleb Natapov wrote:
> > > > On Sun, Apr 22, 2012 at 05:06:43PM +0300, Avi Kivity wrote:
> > > > > On 04/22/2012 04:56 PM, Gleb Natapov wrote:
> > > > > > start. We will need it for migration anyway.
> > > > > >
> > > > > > > hotplug-able memory slots i.e. initial system memory is not 
> > > > > > > modeled with
> > > > > > > memslots. The concept could be generalized to include all memory 
> > > > > > > though, or it
> > > > > > > could more closely follow kvm-memory slots.
> > > > > > OK, I hope final version will allow for memory < 4G to be 
> > > > > > hot-pluggable.
> > > > > 
> > > > > Why is that important?
> > > > > 
> > > > Because my feeling is that people that want to use this kind of feature
> > > > what to start using it with VMs smaller than 4G. Of course not all
> > > > memory have to be hot unpluggable. Making first 1M or event first 128M 
> > > > not
> > > > unpluggable make perfect sense.
> > > 
> > > Can't you achieve this with -m 1G, -device dimm,size=1G,populated=true
> > > -device dimm,size=1G,populated=false?
> > > 
> > From this:
> >
> > (for hw/pc.c PCI hole is currently [below_4g_mem_size, 4G), so
> > hotplugged memory should start from max(4G, above_4g_mem_size).
> >
> > I understand that hotpluggable memory can start from above 4G only. With
> > the config above we will have memory hole from 1G to PCI memory hole.
> > May be not a big problem, but I do not see technical reason for the 
> > constrain.
> >  
> > > (I don't think hotplugging below 512MB is needed, but I don't have any
> > > real data on this).
> > > 
> > 512MB looks like a reasonable limitation too, but again if there is not
> > technical reason for having the limitation why have it?
> >
> 
> I was thinking about not having tons of 128MB slots, so we don't have a
> configuration that is far from reality.  But maybe this thinking is too
> conservative.
> 
I think it is good interface to make memory that is specified with -m to
be one big unpluggable slot, but slots defined with -device should start
just above what -m specifies (after proper alignment). Memory hot-plug
granularity is controlled by slot's size parameter.

--
                        Gleb.