Re: [RFC v1 34/38] target/arm: cpu: only initialize TCG gt timers under CONFIG_TCG

[Alex Bennée]

Claudio Fontana <cfontana@suse.de> writes:

> On 2/22/21 4:34 PM, Alex Bennée wrote:
>> 
>> Claudio Fontana <cfontana@suse.de> writes:
>> 
>>> From: Claudio Fontana <cfontana@centriq4.arch.suse.de>
>>>
>>> KVM has its own cpu->kvm_vtime.
>>>
>>> Adjust cpu vmstate by putting unused fields instead of the
>>> VMSTATE_TIMER_PTR when TCG is not available.
>>>
>>> Signed-off-by: Claudio Fontana <cfontana@suse.de>
>>> ---
>>>  target/arm/cpu.c     | 4 +++-
>>>  target/arm/machine.c | 5 +++++
>>>  2 files changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/target/arm/cpu.c b/target/arm/cpu.c
>>> index 1d81a1e7ac..b929109054 100644
>>> --- a/target/arm/cpu.c
>>> +++ b/target/arm/cpu.c
>>> @@ -1322,6 +1322,7 @@ static void arm_cpu_realizefn(DeviceState *dev, Error 
>>> **errp)
>>>          }
>>>      }
>>>  
>>> +#ifdef CONFIG_TCG
>>>      {
>>>          uint64_t scale;
>>>  
>>> @@ -1347,7 +1348,8 @@ static void arm_cpu_realizefn(DeviceState *dev, Error 
>>> **errp)
>>>          cpu->gt_timer[GTIMER_HYPVIRT] = timer_new(QEMU_CLOCK_VIRTUAL, 
>>> scale,
>>>                                                    arm_gt_hvtimer_cb, cpu);
>>>      }
>>> -#endif
>>> +#endif /* CONFIG_TCG */
>>> +#endif /* !CONFIG_USER_ONLY */
>>>  
>>>      cpu_exec_realizefn(cs, &local_err);
>>>      if (local_err != NULL) {
>>> diff --git a/target/arm/machine.c b/target/arm/machine.c
>>> index 666ef329ef..13d7c6d930 100644
>>> --- a/target/arm/machine.c
>>> +++ b/target/arm/machine.c
>>> @@ -822,8 +822,13 @@ const VMStateDescription vmstate_arm_cpu = {
>>>          VMSTATE_UINT32(env.exception.syndrome, ARMCPU),
>>>          VMSTATE_UINT32(env.exception.fsr, ARMCPU),
>>>          VMSTATE_UINT64(env.exception.vaddress, ARMCPU),
>>> +#ifdef CONFIG_TCG
>>>          VMSTATE_TIMER_PTR(gt_timer[GTIMER_PHYS], ARMCPU),
>>>          VMSTATE_TIMER_PTR(gt_timer[GTIMER_VIRT], ARMCPU),
>>> +#else
>>> +        VMSTATE_UNUSED(sizeof(QEMUTimer *)),
>>> +        VMSTATE_UNUSED(sizeof(QEMUTimer *)),
>>> +#endif /* CONFIG_TCG */
>> 
>> I'm not sure this is correct - VMSTATE_TIMER_PTR chases the links to
>> just expose expired time but QEMUTimer has more in it than that. Paolo
>
>
> I am not sure I follow can you state more precisely where the issue could be?
>
> it's not a VMSTATE_TIMER, it's a VMSTATE_TIMER_PTR,
> it ends up in VMSTATE_POINTER where a single pointer is assigned;

Does it? I thought it ended up with the .expire_time (int64_t) which
will be bigger than sizeof(QemuTimer *) on a 32 bit system.

>
> so if we don't use gt_timer at all (as is the case with !CONFIG_TCG), we just
> need to ensure that an unused number is there to assign, migrating from old 
> to new version?
>
>
>> suggested a straight VMSTATE_UNUSED(8) on IRC but I wonder if it would
>> be better to have a VMSTATE_UNUSED_TIMER?
>> 
>> I don't think there is an impact for Xen because I'm fairly certain
>> migration isn't a thing we do - but I'll double check.
>> 
>
> Thanks Alex, that would be helpful,
> if Xen uses gt_timer in any way I would not want to unwillingly break
> it.

Not for ARM no, currently there is no ARM specific machine emulated by
QEMU for Xen. All ARM guests are PV guests.

>
> Thanks,
>
> Claudio


-- 
Alex Bennée