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Re: ARM Snapshots Not Backwards-Compatible

From: Philippe Mathieu-Daudé
Subject: Re: ARM Snapshots Not Backwards-Compatible
Date: Wed, 3 Feb 2021 16:02:19 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 2/3/21 3:58 PM, Aaron Lindsay wrote:
> On Feb 03 10:01, Peter Maydell wrote:
>>> The third is that meanings of the bits in env->features (as defined by
>>> `enum arm_features` in target/arm/cpu.h) has shifted. For example,
>>> ARM_FEATURE_VFP4 have all been removed and ARM_FEATURE_V8_1M has been
>>> added since 4.1.0. Heck, even I have added a field there in the past.
>>> Unfortunately, these additions/removals mean that when env->features is
>>> saved on one version and restored on another the bits can mean different
>>> things. Notably, the removal of the *VFP features means that a snapshot
>>> of a CPU reporting it supports ARM_FEATURE_VFP3 on 4.1.0 thinks it's now
>>> ARM_FEATURE_M on 5.2.0!
>> Ow. I didn't realize the env->features was in the migration state :-(
>> There is no reason for it to be, because it's a constant property
>> of the CPU. The easy fix is to replace
>>        VMSTATE_UINT64(env.features, ARMCPU),
>> in target/arm/machine.c with whatever the syntax is for "ignore
>> 64 bits of data here". Then we'll ignore whatever is coming in
>> from the source, which we don't need, and we'll stop sending it
>> out if we're the destination.
> I'll look into this.

I think this is:


> -Aaron

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