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Date:   Tue, 06 Mar 2018 10:52:15 +0000
Message-ID: <86woyp4gtc.wl-marc.zyngier@arm.com>
From:   Marc Zyngier <marc.zyngier@arm.com>
To:     Peter Maydell <peter.maydell@linaro.org>
Cc:     Auger Eric <eric.auger@redhat.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 Tue, 06 Mar 2018 10:12:42 +0000,
peter maydell wrote:
> 
> On 6 March 2018 at 09:50, Marc Zyngier <marc.zyngier@arm.com> wrote:
> > On 05/03/18 20:37, Auger Eric wrote:
> >> On 05/03/18 17:31, Peter Maydell wrote:
> >>> That also means that we will fail migration from a new kernel where
> >>> we've specifically asked for PSCI 0.2 to an old PSCI-0.2-only kernel
> >>> (because the KVM_REG_ARM_PSCI_VERSION reg will appear in the migration
> >>> stream even if its value is the one value that matches the old kernel
> >>> behaviour). I don't know if we care about that.
> >>
> >> Do you know when are we likely to force PSCI 0.2 on a new kernel? At
> >> which layer is the decision supposed to be made and on which criteria?
> >
> > No decent SW should need this. But if you've written a guest that cannot
> > work if it doesn't get "2" as response to PSCI_VERSION, you can override it.
> 
> ...but if you want to be able to migrate back from a new kernel to
> an old one, then you need to ask the new kernel for 0.2 so it
> behaves the same way as the old one. As it stands this code wouldn't
> let you do that migration even if you did specifically ask for 0.2.
> (As I said, I don't know if we care about that.)

Absolutely. The moment we introduce a new sysreg, we create a
migration barrier. I'm not sure how the kernel can help in this
respect.

	M.

-- 
Jazz is not dead, it just smell funny.