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

["wangyanan (Y)" <wangyanan55@huawei.com>]

On 2021/4/27 20:36, Philippe Mathieu-Daudé wrote:
> 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.
Hi Philippe,

Of course. I think I can do this for the commit description.

Thanks,
Yanan
> Thanks,
>
> Phil.
>
> .