On Thu, 22 Apr 2021 at 10:55, Philippe Mathieu-Daudé <f4...@amsat.org> wrote: > My guess is CPUState is the only device used in user emulation, > so it would be a way to restrict the vmstate_dummy to CPU and > not to any DeviceState? > > But looking at the introductory commit: > > commit b170fce3dd06372f7bfec9a780ebcb1fce6d57e4 > Author: Andreas Färber <afaer...@suse.de> > Date: Sun Jan 20 20:23:22 2013 +0100 > > cpu: Register VMStateDescription through CPUState > > In comparison to DeviceClass::vmsd, CPU VMState is split in two, > "cpu_common" and "cpu", and uses cpu_index as instance_id instead of -1. > Therefore add a CPU-specific CPUClass::vmsd field. > > Unlike the legacy CPUArchState registration, rather register CPUState. > > Juan, do you remember?
Oh yes, I remember this. There are two ways to handle migration for a CPU object: (1) like any other device, so it has a dc->vmsd that covers migration for the whole object. As usual for objects that are a subclass of a parent that has state, the first entry in the VMStateDescription field list is VMSTATE_CPU(), which migrates the cpu_common fields, followed by whatever the CPU's own migration fields are. (2) a backwards-compatible mechanism for CPUs that were originally migrated using manual "write fields to the migration stream structures". The on-the-wire migration format for those is based on the 'env' pointer (which isn't a QOM object), and the cpu_common part of the migration data is elsewhere. cpu_exec_realizefn() handles both possibilities: * for type 1, dc->vmsd is set and cc->vmsd is not, so cpu_exec_realizefn() does nothing, and the standard "register dc->vmsd for a device" code does everything needed * for type 2, dc->vmsd is NULL and so we register the vmstate_cpu_common directly to handle the cpu-common fields, and the cc->vmsd to handle the per-CPU stuff You can't change a CPU from one type to the other without breaking migration compatibility, which is why some guest architectures are stuck on the cc->vmsd form. New targets should use dc->vmsd. thanks -- PMM