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Re: [Qemu-devel] [RFC PATCH v1 3/3] spapr: spapr: Work around the memory

From: Bharata B Rao
Subject: Re: [Qemu-devel] [RFC PATCH v1 3/3] spapr: spapr: Work around the memory hotplug failure with DDW
Date: Mon, 6 Jun 2016 09:18:38 +0530
User-agent: Mutt/1.5.23 (2014-03-12)

On Fri, Jun 03, 2016 at 05:10:48PM +1000, David Gibson wrote:
> On Fri, Jun 03, 2016 at 11:19:44AM +0530, Bharata B Rao wrote:
> > Memory hotplug can fail for some combinations of RAM and maxmem when
> > DDW is enabled in the presence of devices like nec-usb-xhci. DDW depends
> > on maximum addressable memory returned by guest and this value is currently
> > being calculated wrongly by the guest kernel routine memory_hotplug_max().
> > While there is an attempt to fix the guest kernel, this patch works
> > around the problem within QEMU itself.
> > 
> > memory_hotplug_max() routine in the guest kernel arrives at max
> > addressable memory by multiplying lmb-size with the lmb-count obtained
> > from ibm,dynamic-memory property. There are two assumptions here:
> > 
> > - All LMBs are part of ibm,dynamic memory: This is not true for PowerKVM
> >   where only hot-pluggable LMBs are present in this property.
> > - The memory area comprising of RAM and hotplug region is contiguous: This
> >   needn't be true always for PowerKVM as there can be gap between
> >   boot time RAM and hotplug region.
> > 
> > This work around involves having all the LMBs (RMA, rest of the boot time
> > LMBs and hot-pluggable LMBs) as part of ibm,dynamic-memory so that
> > guest kernel's calculation of max addressable memory comes out correct
> > resulting in correct DDW value which prevents memory hotplug failures.
> > address@hidden is created for RMA, but RMA LMBs are also represented as
> > "reserved" LMBs in ibm,dynamic-memory. Parts of this are essenitally a
> > revert of e8f986fc57a664a74b9f685b466506366a15201b.
> > 
> > In addition to this, the alignment of hotplug memory region is reduced from
> > current 1G to 256M (LMB size in PowerKVM) so that we don't end up with any
> > gaps between boot time RAM and hotplug region.
> Hmm.. could we work around the problem without altering the memory
> alignment by inserting extra dummy reserved LMBs covering the gap?

Appears possible, let me see if it can be done by w/o creating the
DRC objects for such reserved LMBs.


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