Re: ARM Snapshots Not Backwards-Compatible

[Philippe Mathieu-Daudé]

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_PXN, ARM_FEATURE_CRC, ARM_FEATURE_VFP, ARM_FEATURE_VFP3,
>>> 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:

  VMSTATE_UNUSED(sizeof(uint64_t))

> 
> -Aaron
>