* Philippe Mathieu-Daudé (phi...@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.mayd...@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 / dgilb...@redhat.com / Manchester, UK
