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Subject: Re: [REPOST PATCH] arm/arm64: KVM: Add PSCI version selection API
To:     Peter Maydell <peter.maydell@linaro.org>
References: <20180215175803.6870-1-marc.zyngier@arm.com>
 <db215c1e-0ce8-32e4-d131-e790bebf353e@redhat.com>
 <86o9k63f7a.wl-marc.zyngier@arm.com>
 <40c74835-da24-485f-14bb-a0c357c7e79b@redhat.com>
 <CAFEAcA9=YpNngKqeOXiO7QDvL=1ZaQEW8fh3XHjwD+i8qQm9wg@mail.gmail.com>
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
From:   Auger Eric <eric.auger@redhat.com>
Message-ID: <a1579f14-a5fe-11bc-a831-f729cead25ba@redhat.com>
Date:   Mon, 5 Mar 2018 21:37:08 +0100
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Hi Peter,

On 05/03/18 17:31, Peter Maydell wrote:
> On 2 March 2018 at 12:26, Auger Eric <eric.auger@redhat.com> wrote:
>> Hi Marc,
>> On 02/03/18 12:11, Marc Zyngier 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.
>>
>> That's because on the destination, the number of regs is less than on
>> source, right?
> 
> I think it fails because the KVM_REG_ARM_PSCI_VERSION register will be
> in the migration state but not in the destination's list of registers:
> the code in QEMU's target/arm/machine.c:cpu_post_load() function that
> checks "register in their list but not ours: fail migration" will
> catch this.

Thank you for the pointer. Yes at the time I reviewed the patch and just
focusing on the kernel code, this was not immediate to me.

> 
> 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?

Thanks

Eric
> 
> thanks
> -- PMM
>