Re: [Qemu-devel] [RFC v0 1/2] spapr: Accommadate alignment gaps in hotplug memory region

[Igor Mammedov]

On Tue, 6 Oct 2015 14:29:53 +1100
David Gibson <address@hidden> wrote:

> On Mon, Oct 05, 2015 at 11:05:07AM +0200, Igor Mammedov wrote:
> > On Mon,  5 Oct 2015 14:05:23 +0530
> > Bharata B Rao <address@hidden> wrote:
> > 
> > > Size hotplug memory region assuming a 256MB max alignment every slot.
> > > 
> > > Signed-off-by: Bharata B Rao <address@hidden>
> > > ---
> > >  hw/ppc/spapr.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > index fc5e7d6..2ec509b 100644
> > > --- a/hw/ppc/spapr.c
> > > +++ b/hw/ppc/spapr.c
> > > @@ -1792,6 +1792,9 @@ static void ppc_spapr_init(MachineState *machine)
> > >  
> > >          spapr->hotplug_memory.base = ROUND_UP(machine->ram_size,
> > >                                                SPAPR_HOTPLUG_MEM_ALIGN);
> > > +
> > > +        /* size hotplug region assuming 256M max alignment per slot */
> > > +        hotplug_mem_size += SPAPR_MEMORY_BLOCK_SIZE * machine->ram_slots;
> > Does target support hugepages backend? If it does then adjustment probably
> > should be max supported hugepage alignment.
> 
> Hrm, so the maximum possible page size on Power is 16G (though we
> don't yet support that on "powernv" which is what the host system will
> generally be).
> 
> Not sure if the possibility of 16G "colossal pages" in future is
> enough reason to put such a huge gap.  There aren't any other page
> sizes between 16MB and 16GB.

Perhaps add a comment and a more verbose description in commit message
about this so that in future not to wonder why 256 has been chosen.