Re: [Qemu-devel] [RFC PATCH v2 21/23] spapr: Initialize hotplug memory address space

[David Gibson]

On Mon, Apr 13, 2015 at 04:55:52PM +0200, Igor Mammedov wrote:
> On Mon, 13 Apr 2015 19:57:05 +0530
> Bharata B Rao <address@hidden> wrote:
> 
> > On Mon, Apr 13, 2015 at 04:04:24PM +0200, Igor Mammedov wrote:
> > > On Mon, 13 Apr 2015 08:29:33 +0530
> > > Bharata B Rao <address@hidden> wrote:
> > > 
> > > > On Wed, Mar 25, 2015 at 04:58:18PM +1100, David Gibson wrote:
> > > > > On Mon, Mar 23, 2015 at 07:06:02PM +0530, Bharata B Rao wrote:
> > > > > > Initialize a hotplug memory region under which all the hotplugged
> > > > > > memory is accommodated. Also enable memory hotplug by setting
> > > > > > CONFIG_MEM_HOTPLUG.
> > > > > > 
> > > > > > Modelled on i386 memory hotplug.
> > > > > > 
> > > > > > Signed-off-by: Bharata B Rao <address@hidden>
> > > > > > ---
> > > > > >  default-configs/ppc64-softmmu.mak |  1 +
> > > > > >  hw/ppc/spapr.c                    | 50 
> > > > > > +++++++++++++++++++++++++++++++++++++++
> > > > > >  include/hw/ppc/spapr.h            | 12 ++++++++++
> > > > > >  3 files changed, 63 insertions(+)
> > > > > > 
> > > > > > diff --git a/default-configs/ppc64-softmmu.mak 
> > > > > > b/default-configs/ppc64-softmmu.mak
> > > > > > index 22ef132..16b3011 100644
> > > > > > --- a/default-configs/ppc64-softmmu.mak
> > > > > > +++ b/default-configs/ppc64-softmmu.mak
> > > > > > @@ -51,3 +51,4 @@ CONFIG_XICS_KVM=$(and 
> > > > > > $(CONFIG_PSERIES),$(CONFIG_KVM))
> > > > > >  # For PReP
> > > > > >  CONFIG_MC146818RTC=y
> > > > > >  CONFIG_ISA_TESTDEV=y
> > > > > > +CONFIG_MEM_HOTPLUG=y
> > > > > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > > > > index 3e56d9e..e43bb49 100644
> > > > > > --- a/hw/ppc/spapr.c
> > > > > > +++ b/hw/ppc/spapr.c
> > > > > > @@ -125,8 +125,13 @@ struct sPAPRMachineState {
> > > > > >  
> > > > > >      /*< public >*/
> > > > > >      char *kvm_type;
> > > > > > +    ram_addr_t hotplug_memory_base;
> > > > > > +    MemoryRegion hotplug_memory;
> > > > > > +    bool enforce_aligned_dimm;
> > > > > >  };
> > > > > >  
> > > > > > +#define SPAPR_MACHINE_ENFORCE_ALIGNED_DIMM "enforce-aligned-dimm"
> > > > > 
> > > > > What's the rationale for including this option?
> > > > 
> > > > I couldn't see any use, but added it to be similar with x86. May be Igor
> > > > can tell us ?
> > > at least on x86 KVM requires memory region be aligned (otherwise it would 
> > > abort)
> > > and also performance wise it's better to map regions at offsets aligned at
> > > backend page size (especially applicable for hugepages).
> > > 
> > > so region base on x86 is 1Gb aligned to support upto 1Gb hugepages.
> > 
> > My question was specifically about the enforce-aligned-dimm object property
> > that has a get method. I was wondering where this property is consumed. All
> > I could see is that PCMachineState.enforce_aligned_dimm used/referenced
> > directly.
> I think that you don't heed it, it was introduce to to keep memory layout
> compatible with old machine types that didn't have aligned memhp region.
> 
> Since there is nothing to keep compatible in this target, you can just
> drop property and always align region as fit for spapr.

That sounds like a good idea to me.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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