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Date: Mon, 17 Mar 2014 15:48:40 -0700
From: Christoffer Dall <christoffer.dall@linaro.org>
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Subject: Re: [Qemu-devel] [RFC PATCH 5/7] target-arm: Enable
 KVM_ARM_VCPU_PSCI_0_2 feature when possible
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Anup Patel <anup.patel@linaro.org>, patches@apm.com, Alexander Graf <agraf@suse.de>, qemu-devel <qemu-devel@nongnu.org>, "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>, Pranavkumar Sawargaonkar <pranavkumar@linaro.org>

On Mon, Mar 17, 2014 at 07:35:43PM +0000, Peter Maydell wrote:
> [another resend for qemu-devel...]
> 
> On 27 February 2014 07:16, Pranavkumar Sawargaonkar
> <pranavkumar@linaro.org> wrote:
> > On 27 February 2014 12:21, Pranavkumar Sawargaonkar
> > <pranavkumar@linaro.org> wrote:
> >> Latest linux kernel supports in-kernel emulation of PSCI v0.2 but
> >> to enable it we need to select KVM_ARM_VCPU_PSCI_0_2 feature using
> >> KVM_ARM_VCPU_INIT ioctl.
> >>
> >> Also, we can use KVM_ARM_VCPU_PSCI_0_2 feature for VCPU only when
> >> linux kernel has KVM_CAP_ARM_PSCI_0_2 capability.
> >>
> >> This patch updates kvm_arch_init_vcpu() to enable KVM_ARM_VCPU_PSCI_0_2
> >> feature for VCPU when KVM ARM/ARM64 has KVM_CAP_ARM_PSCI_0_2 capability.
> 
> I think this has implications for migration. What happens
> if you migrate a VM from a host system where the host
> kernel supports this capability to one where it doesn't?
> (Presumably the answer is "it won't work" and we need
> to ensure we don't allow that.)
> Conversely, is it OK to migrate a guest from a host
> which didn't support PSCI 0.2 to one which does? Does
> that work even if the destination has PSCI 0.2 enabled
> for the vCPUs, or do we need to support starting a
> VM with 0.2 disabled even on 0.2-capable hosts, for
> use as a migration target?

The init feature should be a switch, turn 0.2 on or off, if off, we're
emulating 0.1.  If 0.2 is on, 0.1 call numbers should generate errors.

You're right, we need to fix this for migration.

> 
> Also we still don't seem to have done anything about
> migration of the PSCI CPU powerup/down state.
> 

We need to create a card for this in Linaro.  Thought I would deal with
it when working through migration for ARMv8.

Thanks!
-Christoffer