Re: [PATCH v3] target/arm/hvf: Fix WFI halting to stop idle vCPU spinning

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

On Tue May 12, 2026 at 1:54 PM EDT, Mohamed Mediouni wrote:
>
>
>> On 27. Apr 2026, at 21:55, 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 per-vCPU QEMU_CLOCK_VIRTUAL 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.
>> 
>> 4. Re-arming the WFI timer from hvf_vm_state_change() on the resume
>>   transition for any halted vCPU, since the QEMUTimer is per-instance
>>   state and is not migrated. After cpu_synchronize_all_states() the
>>   migrated vtimer state is mirrored in env, so we can read CNTV_CTL
>>   and CNTV_CVAL from there. If the vtimer has already expired by the
>>   time the destination resumes, hvf_wfi_timer_cb() is invoked
>>   directly so the halted vCPU is woken up.
>> 
>> Fixes: b5f8f77271 ("accel/hvf: Implement WFI without using pselect()")
>> Signed-off-by: Scott J. Goldman <scottjgo@gmail.com>
>
> Hi,
>
> A bit of a side note for reproducing this:
>
> To reproduce this on current master, as far as I can tell you need either 
> -M kernel-irqchip=off or -M virt-11.0 or earlier. -M virt(-11.1) on master uses 
> the HVF vGIC path.

Just wanted to confirm that I re-tested and it seems like you are
correct. This must have changed semi recently? I was just running with
`-M virt` before. Either way, it's good that the defaults seem to work
better now. Either way, I guess the fix still needs to go in for the
non vGIC path?