Re: ARM Snapshots Not Backwards-Compatible

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Philippe Mathieu-Daudé (philmd@redhat.com) wrote:
> 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))

It's interesting that on x86 we've got a longterm request to *add* cpu
features to the stream to detect screwups caused by using mismatched
CPUs; so it's not necessarily a bad idea to include it once you realise
it's there.

Dave

> > 
> > -Aaron
> > 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK