Re: ARM Snapshots Not Backwards-Compatible

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

* Philippe Mathieu-Daudé (philmd@redhat.com) wrote:
> Cc'ing migration team and qemu-arm@ list.

I'll have to leave the detail of that to the ARM peole; but from a
migration point of view I think we do want the 64 bit ARM migrations to
be stable now.  Please tie incompatible changes to machine types.

Dave

> On 2/3/21 5:01 AM, Aaron Lindsay wrote:
> > Hello,
> > 
> > I'm attempting to restore an AArch64 snapshot taken on QEMU 4.1.0 on
> > QEMU 5.2.0, using system mode. My previous impression, possibly from
> > https://wiki.qemu.org/Features/Migration/Troubleshooting#Basics was that
> > this ought to work:
> > 
> >> Note that QEMU supports migrating forward between QEMU versions
> > 
> > Note that I'm using qemu-system-aarch64 with -loadvm.
> > 
> > However, I've run into several issues I thought I should report. The
> > first of them was that this commit changed the address of CBAR, which
> > resulted in a mismatch of the register IDs in `cpu_post_load` in
> > target/arm/machine.c:
> > https://patchwork.kernel.org/project/qemu-devel/patch/20190927144249.29999-2-peter.maydell@linaro.org/
> > 
> > The second was that several system registers have changed which bits are
> > allowed to be written in different circumstances, seemingly as a result
> > of a combination of bugfixes and implementation of additional behavior.
> > These hit errors detected in `write_list_to_cpustate` in
> > target/arm/helper.c.
> > 
> > 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!
> > 
> > My guess, given the numerous issues and the additional complexity
> > required to properly implement backwards-compatible snapshotting, is
> > that this is not a primary goal. However, if it is a goal, what steps
> > can/should we take to support it more thoroughly?
> > 
> > Thanks!
> > 
> > -Aaron
> > 
> > p.s. Now for an admission: the snapshots I'm testing with were
> > originally taken with `-cpu max`. This was unintentional, and I
> > understand if the response is that I can't expect `-cpu max` checkpoints
> > to work across QEMU versions... but I also don't think that all of these
> > issues can be blamed on that (notably CBAR and env->features).
> > 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK