> From: Igor Mammedov <imamm...@redhat.com>
> Sent: Monday, August 12, 2024 9:16 AM
> To: Gavin Shan <gs...@redhat.com>
>
> On Mon, 12 Aug 2024 14:35:56 +1000
> Gavin Shan <gs...@redhat.com> wrote:
>
> > On 6/14/24 9:36 AM, Salil Mehta wrote:
> > > This shall be used to store user specified
> > > topology{socket,cluster,core,thread}
> > > and shall be converted to a unique 'vcpu-id' which is used as
> > > slot-index during hot(un)plug of vCPU.
> > >
> > > Co-developed-by: Keqian Zhu <zhukeqi...@huawei.com>
> > > Signed-off-by: Keqian Zhu <zhukeqi...@huawei.com>
> > > Signed-off-by: Salil Mehta <salil.me...@huawei.com>
> > > ---
> > > hw/arm/virt.c | 10 ++++++++++
> > > include/hw/arm/virt.h | 28 ++++++++++++++++++++++++++++
> > > target/arm/cpu.c | 4 ++++
> > > target/arm/cpu.h | 4 ++++
> > > 4 files changed, 46 insertions(+)
> > >
> >
> > Those 4 properties are introduced to determine the vCPU's slot, which
> > is the index to MachineState::possible_cpus::cpus[]. From there, the
> > CPU object or instance is referenced and then the CPU's state can be
> > further determined. It sounds reasonable to use the CPU's topology to
> > determine the index. However, I'm wandering if this can be simplified to
> use 'cpu-index' or 'index' for a couple of facts: (1) 'cpu-index'
>
> Please, don't. We've spent a bunch of time to get rid of cpu-index in user
> visible interface (well, old NUMA CLI is still there along with 'new'
> topology
> based one, but that's the last one).
Agreed. We shouldn't expose CPU index to user.
>
> > or 'index' is simplified. Users have to provide 4 parameters in order
> > to determine its index in the extreme case, for example "device_add
> > host-arm-cpu, id=cpu7,socket-id=1,
> > cluster-id=1,core-id=1,thread-id=1". With 'cpu-index' or 'index', it
> > can be simplified to 'index=7'. (2) The cold-booted and hotpluggable
> > CPUs are determined by their index instead of their topology. For
> > example, CPU0/1/2/3 are cold-booted CPUs while CPU4/5/6/7 are
> hotpluggable CPUs with command lines '-smp maxcpus=8,cpus=4'. So 'index'
> makes more sense to identify a vCPU's slot.
> cpu-index have been used for hotplug with x86 machines as a starting point
> to implement hotplug as it was easy to hack and it has already existed in
> QEMU.
>
> But that didn't scale as was desired and had its own issues.
> Hence the current interface that majority agreed upon.
> I don't remember exact arguments anymore (they could be found qemu-
> devel if needed) Here is a link to the talk that tried to explain why topo
> based was introduced.
>
> http://events17.linuxfoundation.org/sites/events/files/slides/CPU%20Hot-
> plug%20support%20in%20QEMU.pdf
I think you are referring to slide-19 of above presentation?
Thanks
Salil.
