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Date:   Tue, 6 Mar 2018 10:21:34 +0100
From:   Andrew Jones <drjones@redhat.com>
To:     Peter Maydell <peter.maydell@linaro.org>
Cc:     Marc Zyngier <marc.zyngier@arm.com>,
        lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
        arm-mail-list <linux-arm-kernel@lists.infradead.org>,
        kvmarm@lists.cs.columbia.edu
Subject: Re: [REPOST PATCH] arm/arm64: KVM: Add PSCI version selection API
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On Mon, Mar 05, 2018 at 04:47:55PM +0000, Peter Maydell wrote:
> On 2 March 2018 at 11:11, Marc Zyngier <marc.zyngier@arm.com> wrote:
> > On Fri, 02 Mar 2018 10:44:48 +0000,
> > Auger Eric wrote:
> >> I understand the get/set is called as part of the migration process.
> >> So my understanding is the benefit of this series is migration fails in
> >> those cases:
> >>
> >> >=0.2 source -> 0.1 destination
> >> 0.1 source -> >=0.2 destination
> >
> > It also fails in the case where you migrate a 1.0 guest to something
> > that cannot support it.
> 
> I think it would be useful if we could write out the various
> combinations of source, destination and what we expect/want to
> have happen. My gut feeling here is that we're sacrificing
> exact migration compatibility in favour of having the guest
> automatically get the variant-2 mitigations, but it's not clear
> to me exactly which migration combinations that's intended to
> happen for. Marc?
> 
> If this wasn't a mitigation issue the desired behaviour would be
> straightforward:
>  * kernel should default to 0.2 on the basis that
>    that's what it did before
>  * new QEMU version should enable 1.0 by default for virt-2.12
>    and 0.2 for virt-2.11 and earlier
>  * PSCI version info shouldn't appear in migration stream unless
>    it's something other than 0.2
> But that would leave some setups (which?) unnecessarily without the
> mitigation, so we're not doing that. The question is, exactly
> what *are* we aiming for?

The reason Marc dropped this patch from the series it was first introduced
in was because we didn't have the aim 100% understood. We want the
mitigation by default, but also to have the least chance of migration
failure, and when we must fail (because we're not doing the
straightforward approach listed above, which would prevent failures), then
we want to fail with the least amount of damage to the user.

I experimented with a couple different approaches and provided tables[1]
with my results. I even recommended an approach, but I may have changed
my mind after reading Marc's follow-up[2]. The thread continues from
there as well with follow-ups from Christoffer, Marc, and myself. Anyway,
Marc did this repost for us to debate it and work out the best approach
here.

Thanks,
drew

[1] https://www.spinics.net/lists/arm-kernel/msg632355.html
[2] https://www.spinics.net/lists/arm-kernel/msg632385.html