Re: [PATCH] cpu: Add starts_halted() method

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

Hi Thiago,

On 7/8/20 1:28 AM, Thiago Jung Bauermann wrote:
> 
> Hello Eduardo,
> 
> Eduardo Habkost <ehabkost@redhat.com> writes:
> 
>> On Tue, Jul 07, 2020 at 05:43:33PM -0300, Thiago Jung Bauermann wrote:
>>> PowerPC sPAPRs CPUs start in the halted state, but generic QEMU code
>>> assumes that CPUs start in the non-halted state. spapr_reset_vcpu()
>>> attempts to rectify this by setting CPUState::halted to 1. But that's too
>>> late for hotplugged CPUs in a machine configured with 2 or mor threads per
>>> core.
>>>
>>> By then, other parts of QEMU have already caused the vCPU to run in an
>>> unitialized state a couple of times. For example, ppc_cpu_reset() calls
>>> ppc_tlb_invalidate_all(), which ends up calling async_run_on_cpu(). This
>>> kicks the new vCPU while it has CPUState::halted = 0, causing QEMU to issue
>>> a KVM_RUN ioctl on the new vCPU before the guest is able to make the
>>> start-cpu RTAS call to initialize its register state.
>>>
>>> This doesn't seem to cause visible issues for regular guests, but on a
>>> secure guest running under the Ultravisor it does. The Ultravisor relies on
>>> being able to snoop on the start-cpu RTAS call to map vCPUs to guests, and
>>> this issue causes it to see a stray vCPU that doesn't belong to any guest.
>>>
>>> Fix by adding a starts_halted() method to the CPUState class, and making it
>>> return 1 if the machine is an sPAPR guest.
>>>
>>> Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
>> [...]
>>> +static uint32_t ppc_cpu_starts_halted(void)
>>> +{
>>> +    SpaprMachineState *spapr =
>>> +        (SpaprMachineState *) object_dynamic_cast(qdev_get_machine(),
>>> +                                                  TYPE_SPAPR_MACHINE);
>>
>> Wouldn't it be simpler to just implement this as a MachineClass
>> boolean field?  e.g.:

Class boolean field certainly sounds better, but I am not sure this
is a property of the machine. Rather the arch? So move the field
to CPUClass? Maybe not, let's discuss :)

>>
>> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> 
> Yes, indeed it would. Thanks for this patch. I just tested and it
> also solves the problem (except for the nit mentioned below).
> 
> Tested-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
> 
> Should I submit a proper patch with these changes (with you as the
> author)?
> 
>> ---
>> diff --git a/include/hw/boards.h b/include/hw/boards.h
>> index 426ce5f625..ffadc7a17d 100644
>> --- a/include/hw/boards.h
>> +++ b/include/hw/boards.h
>> @@ -215,6 +215,7 @@ struct MachineClass {
>>      bool nvdimm_supported;
>>      bool numa_mem_supported;
>>      bool auto_enable_numa;
>> +    bool cpu_starts_halted;
>>      const char *default_ram_id;
>>
>>      HotplugHandler *(*get_hotplug_handler)(MachineState *machine,
>> diff --git a/hw/core/cpu.c b/hw/core/cpu.c
>> index 0f23409f1d..08dd504034 100644
>> --- a/hw/core/cpu.c
>> +++ b/hw/core/cpu.c
>> @@ -252,6 +252,7 @@ static void cpu_common_reset(DeviceState *dev)
>>  {
>>      CPUState *cpu = CPU(dev);
>>      CPUClass *cc = CPU_GET_CLASS(cpu);
>> +    MachineState *machine = object_dynamic_cast(qdev_get_machine(), TYPE_MACHINE);
> 
> I had to add a (MachineState *) cast here to get the code to compile.

Btw why not use MACHINE(qdev_get_machine()) ?

> 
>>
>>      if (qemu_loglevel_mask(CPU_LOG_RESET)) {
>>          qemu_log("CPU Reset (CPU %d)\n", cpu->cpu_index);
>> @@ -259,7 +260,7 @@ static void cpu_common_reset(DeviceState *dev)
>>      }
>>
>>      cpu->interrupt_request = 0;
>> -    cpu->halted = 0;
>> +    cpu->halted = machine ? MACHINE_GET_CLASS(machine)->cpu_starts_halted : 0;
>>      cpu->mem_io_pc = 0;
>>      cpu->icount_extra = 0;
>>      atomic_set(&cpu->icount_decr_ptr->u32, 0);
>> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
>> index f6f034d039..d16ec33033 100644
>> --- a/hw/ppc/spapr.c
>> +++ b/hw/ppc/spapr.c
>> @@ -4487,6 +4487,7 @@ static void spapr_machine_class_init(ObjectClass *oc, void *data)
>>      mc->default_cpu_type = POWERPC_CPU_TYPE_NAME("power9_v2.0");
>>      mc->has_hotpluggable_cpus = true;
>>      mc->nvdimm_supported = true;
>> +    mc->cpu_starts_halted = true;
>>      smc->resize_hpt_default = SPAPR_RESIZE_HPT_ENABLED;
>>      fwc->get_dev_path = spapr_get_fw_dev_path;
>>      nc->nmi_monitor_handler = spapr_nmi;
>>
>>> +
>>> +    /*
>>> +     * In sPAPR, all CPUs start halted. CPU0 is unhalted from the machine level
>>> +     * reset code and the rest are explicitly started up by the guest using an
>>> +     * RTAS call.
>>> +     */
>>> +    return spapr != NULL;
>>> +}
>>> +
> 
> 
> --
> Thiago Jung Bauermann
> IBM Linux Technology Center
>