Thanks everyone. It sounds like there is consensus around how best to
proceed (at a high level at least).
Since Igor has already gotten started, I'll coordinate with him offline
to see where I can jump in.
Thanks,
-Maran
On 7/25/2018 6:07 AM, Marc Zyngier wrote:
On 25/07/18 13:28, Andrew Jones wrote:
On Wed, Jul 25, 2018 at 11:40:54AM +0100, Marc Zyngier wrote:
On 24/07/18 19:35, Maran Wilson wrote:
It's been a few months since this email thread died off. Has anyone
started working on a potential solution that would allow VCPU hotplug on
KVM/ARM ? Or is this a project that is still waiting for an owner who
has the time and inclination to get started?
This is typically a project for someone who would have this particular
itch to scratch, and who has a demonstrable need for this functionality.
Work wise, it would have to include adding physical CPU hotplug support
to the arm64 kernel as a precondition, before worrying about doing it in
KVM.
For KVM itself, particular area of interests would be:
- Making GICv3 redistributors magically appear in the IPA space
- Live resizing of GICv3 structures
- Dynamic allocation of MPIDR, and mapping with vcpu_id
I have CPU topology description patches on the QEMU list now[*]. A next
step for me is to this MPIDR work. I probably won't get to it until the
end of August though.
[*] http://lists.nongnu.org/archive/html/qemu-devel/2018-07/msg01168.html
This should keep someone busy for a good couple of weeks (give or take a
few months).
:-)
That being said, I'd rather see support in QEMU first, creating all the
vcpu/redistributors upfront, and signalling the hotplug event via the
virtual firmware. And then post some numbers to show that creating all
the vcpus upfront is not acceptable.
I think the upfront allocation, allocating all possible cpus, but only
activating all present cpus, was the planned approach. What were the
concerns about that approach? Just vcpu memory overhead for too many
overly ambitious VM configs?
I don't have any ARM-specific concern about that, and I think this is
the right approach. It has the good property of not requiring much
change in the kernel (other than actually supporting CPU hotplug).
vcpu memory overhead is a generic concern though, and not only for ARM.
We currently allow up to 512 vcpus per VM, which looks like a lot, but
really isn't. If we're to allow this to be bumped up significantly, we
should start accounting the vcpu-related memory against the user's
allowance...
Thanks,
M.
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