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Re: [Qemu-devel] [Qemu-arm] [PATCH v2 2/2] arm_gicv3_kvm: kvm_dist_get/p

From: Peter Maydell
Subject: Re: [Qemu-devel] [Qemu-arm] [PATCH v2 2/2] arm_gicv3_kvm: kvm_dist_get/put: skip the registers banked by GICR
Date: Thu, 5 Apr 2018 15:22:12 +0100

On 29 March 2018 at 11:54, Peter Maydell <address@hidden> wrote:
> On 23 March 2018 at 12:08, Peter Maydell <address@hidden> wrote:
>> On 21 March 2018 at 08:00, Shannon Zhao <address@hidden> wrote:
>>> On 2018/3/20 19:54, Peter Maydell wrote:
>>>> Can you still successfully migrate a VM from a QEMU version
>>>> without this bugfix to one with the bugfix ?
>>> I've tested this case. I can migrate a VM between these two versions.
>> Hmm. Looking at the code I can't see how that would work,
>> except by accident. Let me see if I understand what's happening
>> here:

> I was thinking a bit more about how to handle this, and
> my best idea was:
> (1) send something in the migration stream that says
> "I don't have this bug" (version number change?
> vmstate field that's just a "no bug" flag? subsection
> with no contents?)
> (2) on the destination, if the source doesn't tell us
> it doesn't have this bug, and we are running KVM, then
> shift all the data in the arrays down to fix it up
> [Strictly what we want to know is if the source is
> running KVM, not if the destination is, but I don't
> know of a way to find that out, and in practice TCG->KVM
> migrations don't work anyway, so it's not a big deal.]

Shannon, are you planning to look at this for 2.12, or should
we postpone it to 2.13? (It's not a regression, right? So
we don't necessarily have to urgently fix it for 2.12.)

-- PMM

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