Re: [PATCH v5 03/28] pc: Start with modern CPU hotplug interface by default

[Zhao Liu <zhao1.liu@intel.com>]

On Wed, Dec 17, 2025 at 02:32:37PM +0100, Igor Mammedov wrote:
> Date: Wed, 17 Dec 2025 14:32:37 +0100
> From: Igor Mammedov <imammedo@redhat.com>
> Subject: Re: [PATCH v5 03/28] pc: Start with modern CPU hotplug interface
>  by default
> X-Mailer: Claws Mail 4.3.1 (GTK 3.24.51; x86_64-redhat-linux-gnu)
> 
> On Wed,  3 Dec 2025 00:28:10 +0800
> Zhao Liu <zhao1.liu@intel.com> wrote:
> 
> > From: Igor Mammedov <imammedo@redhat.com>
> ^^^
> given you resplit original patch, it's better to replace this with you,
> keeping my SoB is sufficient

Thank you! Will re-organize these signatures

> > For compatibility reasons PC/Q35 will start with legacy CPU hotplug
> > interface by default but with new CPU hotplug AML code since 2.7
> > machine type (in commit 679dd1a957df ("pc: use new CPU hotplug interface
> > since 2.7 machine type")). In that way, legacy firmware that doesn't use
> > QEMU generated ACPI tables was able to continue using legacy CPU hotplug
> > interface.
> > 
> > While later machine types, with firmware supporting QEMU provided ACPI
> > tables, generate new CPU hotplug AML, which will switch to new CPU
> > hotplug interface when guest OS executes its _INI method on ACPI tables
> > loading.
> > 
> > Since 2.6 machine type is now gone, and consider that the legacy BIOS
> > (based on QEMU ACPI prior to v2.7) should be no longer in use, previous
> > compatibility requirements are no longer necessary. So initialize
> > 'modern' hotplug directly from the very beginning for PC/Q35 machines
> > with cpu_hotplug_hw_init(), and drop _INIT method.
> > 
> > Additionally, remove the checks and settings around cpu_hotplug_legacy
> > in cpuhp VMState (for piix4 & ich9), to eliminate the risk of
> > segmentation faults, as gpe_cpu no longer has the opportunity to be
> > initialized. This is safe because all hotplug now start with the modern
> > way, and it's impossible to switch to legacy way at runtime (even the
> > "cpu-hotplug-legacy" properties does not allow it either).
> > 
> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> 
> tested ping pong cross version (master vs master+this patch) migration
> with 10.1 machine type, nothing is broken, hence
> 
> Acked-by: Igor Mammedov <imammedo@redhat.com>

Thanks for your test and review!

Regards,
Zhao