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Re: [Qemu-ppc] [RFC PATCH v2 00/23] CPU and Memory hotplug for PowerPC s


From: Bharata B Rao
Subject: Re: [Qemu-ppc] [RFC PATCH v2 00/23] CPU and Memory hotplug for PowerPC sPAPR guests
Date: Mon, 6 Apr 2015 15:49:24 +0530
User-agent: Mutt/1.5.23 (2014-03-12)

On Mon, Mar 23, 2015 at 07:05:41PM +0530, Bharata B Rao wrote:
> Hi,
> 
> This is the version 2 of the patchset that provides CPU and memory hotplug
> support for PowerPC sPAPR guests.

[snip]

> TODOs
> -----
> - Share code between pc_dimm_plug() and spapr_memory_plug().
> - Make the algorithm that looks up the NUMA node given the physical address
>   more efficient.
> - Test/enable migration after hotplug.

While I am using a bitmap based CPU enumeration (patch 14/23 in this patchset),
to correctly support CPU hot removal, it appears that supporting hot removal
with migration is non trivial if we allow removal in arbitrary order and
not necessarily remove-last-added-cpu-first order.

If there are holes in CPU index map (like 0-15,20-23) at the source VM
due to hot removal, the CPU index map at the target VM will be 0-19 by
default.

I see there was a patch to solve somewhat similar problem on x86 last year
(https://lists.nongnu.org/archive/html/qemu-devel/2014-01/msg01607.html), but
don't see it being pursued further.

What would be the ideal or recommended way to solve this problem ? Easiest
would be to enforce hot removal in LIFO order, but I guess that would be
restrictive.

Regards,
Bharata.




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