Re: [RFC PATCH v2 2/6] hw/arm/virt: DT: Add cpu-map

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 4/27/21 12:04 PM, Andrew Jones wrote:
> On Tue, Apr 27, 2021 at 11:47:17AM +0200, Philippe Mathieu-Daudé wrote:
>> Hi Yanan, Drew,
>>
>> On 4/13/21 10:07 AM, Yanan Wang wrote:
>>> From: Andrew Jones <drjones@redhat.com>
>>>
>>> Support device tree CPU topology descriptions.
>>>
>>> Signed-off-by: Andrew Jones <drjones@redhat.com>
>>> Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
>>> ---
>>>  hw/arm/virt.c         | 41 ++++++++++++++++++++++++++++++++++++++++-
>>>  include/hw/arm/virt.h |  1 +
>>>  2 files changed, 41 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
>>> index 9f01d9041b..f4ae60ded9 100644
>>> --- a/hw/arm/virt.c
>>> +++ b/hw/arm/virt.c
>>> @@ -352,10 +352,11 @@ static void fdt_add_cpu_nodes(const VirtMachineState *vms)
>>>      int cpu;
>>>      int addr_cells = 1;
>>>      const MachineState *ms = MACHINE(vms);
>>> +    const VirtMachineClass *vmc = VIRT_MACHINE_GET_CLASS(vms);
>>>      int smp_cpus = ms->smp.cpus;
>>>  
>>>      /*
>>> -     * From Documentation/devicetree/bindings/arm/cpus.txt
>>> +     *  See Linux Documentation/devicetree/bindings/arm/cpus.yaml
>>>       *  On ARM v8 64-bit systems value should be set to 2,
>>>       *  that corresponds to the MPIDR_EL1 register size.
>>>       *  If MPIDR_EL1[63:32] value is equal to 0 on all CPUs
>>> @@ -408,8 +409,45 @@ static void fdt_add_cpu_nodes(const VirtMachineState *vms)
>>>                  ms->possible_cpus->cpus[cs->cpu_index].props.node_id);
>>>          }
>>>  
>>> +        if (ms->smp.cpus > 1 && !vmc->no_cpu_topology) {
>>> +            qemu_fdt_setprop_cell(ms->fdt, nodename, "phandle",
>>> +                                  qemu_fdt_alloc_phandle(ms->fdt));
>>> +        }
>>> +
>>>          g_free(nodename);
>>>      }
>>> +
>>> +    if (ms->smp.cpus > 1 && !vmc->no_cpu_topology) {
>>> +        /*
>>> +         * See Linux Documentation/devicetree/bindings/cpu/cpu-topology.txt
>>> +         * In a SMP system, the hierarchy of CPUs is defined through four
>>> +         * entities that are used to describe the layout of physical CPUs
>>> +         * in the system: socket/cluster/core/thread.
>>> +         */
>>> +        qemu_fdt_add_subnode(ms->fdt, "/cpus/cpu-map");
>>> +
>>> +        for (cpu = ms->smp.cpus - 1; cpu >= 0; cpu--) {
>>> +            char *cpu_path = g_strdup_printf("/cpus/cpu@%d", cpu);
>>> +            char *map_path;
>>> +
>>> +            if (ms->smp.threads > 1) {
>>> +                map_path = g_strdup_printf(
>>> +                    "/cpus/cpu-map/%s%d/%s%d/%s%d",
>>> +                    "socket", cpu / (ms->smp.cores * ms->smp.threads),
>>> +                    "core", (cpu / ms->smp.threads) % ms->smp.cores,
>>> +                    "thread", cpu % ms->smp.threads);
>>> +            } else {
>>> +                map_path = g_strdup_printf(
>>> +                    "/cpus/cpu-map/%s%d/%s%d",
>>> +                    "socket", cpu / ms->smp.cores,
>>> +                    "core", cpu % ms->smp.cores);
>>> +            }
>>> +            qemu_fdt_add_path(ms->fdt, map_path);
>>> +            qemu_fdt_setprop_phandle(ms->fdt, map_path, "cpu", cpu_path);
>>> +            g_free(map_path);
>>> +            g_free(cpu_path);
>>> +        }
>>> +    }
>>>  }
>>>  
>>>  static void fdt_add_its_gic_node(VirtMachineState *vms)
>>> @@ -2769,6 +2807,7 @@ static void virt_machine_5_2_options(MachineClass *mc)
>>>      virt_machine_6_0_options(mc);
>>>      compat_props_add(mc->compat_props, hw_compat_5_2, hw_compat_5_2_len);
>>>      vmc->no_secure_gpio = true;
>>> +    vmc->no_cpu_topology = true;
>>
>> Bare with me because "machine versioning" is something new to me, I was
>> expecting it to be only related to migrated fields.
>> Why do we need to care about not adding the FDT node in older machines?
>> Shouldn't the guest skip unknown FDT nodes?
> 
> It probably should, the question is whether it would. Also, the nodes may
> not be unknown, so the guest will read the information and set up its
> topology as instructed. That topology may not be the same as what was
> getting used by default without the topology description. It's possible
> that a user's application has a dependency on the topology and if that
> topology gets changed under its feat it'll behave differently.

[*]

I see.

> In short, machine versioning isn't just about vmstate, it's also about
> keeping a machine type looking the same to the guest.

Yes, TIL.

> Now, it's possible that we're being overly cautious here, but this compat
> variable doesn't complicate code too much. So I think I'd prefer to use it
> than not.

No problem. Could you or Yanan add your first paragraph ([*], reworded
in the commit description? I don't think a comment in the code is
useful, but having it in the commit is helpful IMO.

Thanks,

Phil.