Re: [Qemu-stable] [PATCH] x86: do not advertise die-id in query-hotpluggbale-cpus if '-smp dies' is not set

[Eduardo Habkost]

On Mon, Sep 02, 2019 at 08:02:22AM -0400, Igor Mammedov wrote:
> Commit 176d2cda0 (i386/cpu: Consolidate die-id validity in smp context) added
> new 'die-id' topology property to CPUs and exposed it via QMP command
> query-hotpluggable-cpus, which broke -device/device_add cpu-foo for existing
> users that do not support die-id/dies yet. That's would be fine if it happened
> to new machine type only but it also happened to old machine types,
> which breaks migration from old QEMU to the new one, for example following 
> CLI:
> 
>   OLD-QEMU -M pc-i440fx-4.0 -smp 1,max_cpus=2 \
>            -device qemu64-x86_64-cpu,socket-id=1,core-id=0,thread-id
> is not able to start with new QEMU, complaining about invalid die-id.
> 
> After discovering regression, the patch
>    "pc: Don't make die-id mandatory unless necessary"
> makes die-id optional so old CLI would work.
> 
> However it's not enough as new QEMU still exposes die-id via 
> query-hotpluggbale-cpus
> QMP command, so the users that started old machine type on new QEMU, using all
> properties (including die-id) received from QMP command (as required), won't 
> be
> able to start old QEMU using the same properties since it doesn't support 
> die-id.
> 
> Fix it by hiding die-id in query-hotpluggbale-cpus for all machine types in 
> case
> '-smp dies' is not provided on CLI or -smp dies = 1', in which case smp_dies 
> == 1
> and APIC ID is calculated in default way (as it was before DIE support) so we 
> won't
> need compat code as in both cases the topology provided to guest via CPUID is 
> the same.
> 
> Signed-off-by: Igor Mammedov <address@hidden>

Reviewed-by: Eduardo Habkost <address@hidden>

Queued on machine-next.

-- 
Eduardo