Re: [PATCH v2] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

["Scott J. Goldman" <scottjgo@gmail.com>]

On Fri Apr 17, 2026 at 1:30 PM PDT, Scott J. Goldman wrote:
> On Fri Apr 17, 2026 at 2:57 AM PDT, Philippe Mathieu-Daudé wrote:
>> Hi Scott,
>>
>> On 16/4/26 23:20, Scott J. Goldman wrote:
>>> Philippe-- apologies for the nag here but this is a fix for a unfortunate
>>> regression. Any chance you could take a look? Thanks!
>>> 
>>> On Thu Apr 9, 2026 at 11:18 PM PDT, Mohamed Mediouni wrote:
>>>>
>>>>
>>>>> On 10. Apr 2026, at 07:50, Scott J. Goldman <scottjgo@gmail.com> wrote:
>>>>>
>>>>> Commit b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>>>> changed hvf_wfi() from blocking the vCPU thread with pselect() to
>>>>> returning EXCP_HLT, intending QEMU's main event loop to handle the
>>>>> idle wait. However, cpu->halted was never set, so cpu_thread_is_idle()
>>>>> always returns false and the vCPU thread spins at 100% CPU per core
>>>>> while the guest is idle.
>>>>>
>>>>> Fix this by:
>>>>>
>>>>> 1. Setting cpu->halted = 1 in hvf_wfi() so the vCPU thread sleeps on
>>>>>    halt_cond in qemu_process_cpu_events().
>>>>>
>>>>> 2. Arming a host-side QEMU_CLOCK_HOST timer to fire when the guest's
>>>>>    virtual timer (CNTV_CVAL_EL0) would expire. This is necessary
>>>>>    because HVF only delivers HV_EXIT_REASON_VTIMER_ACTIVATED during
>>>>>    hv_vcpu_run(), which is not called while the CPU is halted. The
>>>>>    timer callback mirrors the VTIMER_ACTIVATED handler: it raises the
>>>>>    vtimer IRQ through the GIC and marks vtimer_masked, causing the
>>>>>    interrupt delivery chain to wake the vCPU via qemu_cpu_kick().
>>>>>
>>>>> 3. Clearing cpu->halted in hvf_arch_vcpu_exec() when cpu_has_work()
>>>>>    indicates a pending interrupt, and cancelling the WFI timer.
>>>>>
>>>>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>>>>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>>>>
>>>> Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
>>>>> ---
>>>>> include/system/hvf_int.h |  1 +
>>>>> target/arm/hvf/hvf.c     | 65 +++++++++++++++++++++++++++++++++++++++-
>>>>> 2 files changed, 65 insertions(+), 1 deletion(-)
>>
>> I haven't looked at your patch yet. While looking at commit
>> b5f8f77271 I noticed the previous a14afa985e3 ("accel/hvf: Skip
>> WFI if CPU has work to do") is different in my local tree, where
>> I have:
>>
>> -- >8 --
>> diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
>> index 5fc8f6bbbd9..1a76c9cf402 100644
>> --- a/target/arm/hvf/hvf.c
>> +++ b/target/arm/hvf/hvf.c
>> @@ -2037,6 +2037,11 @@ static int hvf_wfi(CPUState *cpu)
>>           return 0;
>>       }
>>
>> +    if (cpu_test_interrupt(cpu, CPU_INTERRUPT_HARD | CPU_INTERRUPT_FIQ)) {
>> +        /* Interrupt pending, no need to wait */
>> +        return EXCP_INTERRUPT;
>> +    }
>> +
>>       return EXCP_HLT;
>>   }
>>
>> ---
>>
>> Does that help? Now I'm really confused.
>
>
> Hi Phillipe-- Appreciate your help here. The good news is the bug is
> very easy to reproduce (start a linux VM with 1 cpu, watch the vCPU on
> the host will always spin 100%). The bad news is I double checked my
> bisect and you can see before b5f8f77271 there is no problem, and after
> the commit is introduced, the problem appears. I also applied the
> additional cpu_test_interrupt() patch you suggested and it did not
> change the behavior.
>
> thanks
> -sjg

Hi Philippe- Apologies again for bugging you, but it looks like v11 is
about to be released and it would be a bummer if this regression went
out with the stable version. Have you had a chance to either look at my
patch or check if there was some other simpler fix?

Also, I'm not a regular qemu contributor, so if there is some process
here, that I'm missing, please let me know.

thanks
-sjg