Re: [PATCH v3 03/17] clocksource/drivers/arm_arch_timer: Default to EL2 virtual timer when running VHE

[Marc Zyngier <maz@kernel.org>]

On Tue, 09 Jun 2026 11:35:24 +0100,
Marek Szyprowski <m.szyprowski@samsung.com> wrote:
> 
> On 09.06.2026 12:21, Marc Zyngier wrote:
> > On Tue, 09 Jun 2026 11:03:21 +0100,
> > Marek Szyprowski <m.szyprowski@samsung.com> wrote:
> >> On 23.05.2026 16:02, Marc Zyngier wrote:
> >>> When running with at EL2 with VHE enabled, the architecture provides
> >>> two EL2 timer/counters, dubbed physical and virtual. Apart from their
> >>> names, they are strictly identical.
> >>>
> >>> However, they don't get virtualised the same way, specially when
> >>> it comes to adding arbitrary offsets to the timers. When running as
> >>> a guest, the host CNTVOFF_EL2 does apply to the guest's view of
> >>> CNTHV*_El2. This is not true for CNTPOFF_EL2 and CNTHP*_EL2, as
> >>> the architecture is broken past the first level of virtualisation
> >>> (it lacks some essential mechanisms to be usable, despite what
> >>> the ARM ARM pretends).
> >>>
> >>> This means that when running as a L2 guest hypervisor, using the
> >>> physical timer results in traps to L0, which are then forwarded to
> >>> L1 in order to emulate the offset, leading to even worse performance
> >>> due to massive trap amplification (the combination of register and
> >>> ERET trapping is absolutely lethal).
> >>>
> >>> Switch the arch timer code to using the virtual timer when running
> >>> in VHE by default, only using the physical timer if the interrupt
> >>> is not correctly described in the firmware tables (which seems
> >>> to be an unfortunately common case). This comes as no impact on
> >>> bare-metal, and slightly improves the situation in the virtualised
> >>> case.
> >>>
> >>> Signed-off-by: Marc Zyngier <maz@kernel.org>
> >> This patch landed recently in linux-next as commit d87773de9efe
> >> ("clocksource/drivers/arm_arch_timer: Default to EL2 virtual timer when
> >> running VHE"). In my tests I found that it breaks booting of RaspberryPi5
> >> board. Reverting it on top of linux-next fixes the issue. Here is a boot
> >> log:
> > Huh.
> >
> > [...]
> >
> >> arch_timer: cp15 timer running at 54.00MHz (hyp-virt).
> >> clocksource: arch_sys_counter: mask: 0xffffffffffffff max_cycles: 0xc743ce346, max_idle_ns: 440795203123 ns
> >> sched_clock: 56 bits at 54MHz, resolution 18ns, wraps every 4398046511102ns
> > The interrupt appears to be advertised in the DT, but doesn't seem to
> > fire. That's obviously not going to end well. My suspicion is that
> > either the interrupt isn't wired (that'd be hilariously abd), or is
> > left as Group-0 by the firmware (copy-paste from RPi4).
> >
> > Can you try the following hack and let me know if the kernel shouts at
> > you?
> >
> > Thanks,
> >
> > 	M.
> >
> > diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
> > index ec70c84e9f91d..d05791e6cc0db 100644
> > --- a/drivers/irqchip/irq-gic.c
> > +++ b/drivers/irqchip/irq-gic.c
> > @@ -213,6 +213,7 @@ static void gic_eoimode1_mask_irq(struct irq_data *d)
> >  static void gic_unmask_irq(struct irq_data *d)
> >  {
> >  	gic_poke_irq(d, GIC_DIST_ENABLE_SET);
> > +	WARN_ON(!gic_peek_irq(d, GIC_DIST_ENABLE_SET));
> >  }
> >  
> >  static void gic_eoi_irq(struct irq_data *d)
> 
> I've applied this change, but it doesn't trigger any warning in the boot log.

[+ Florian]

Huh. So that really points at the timer not being wired into the GIC,
Samsung style... Can you confirm that removing the EL2 virtual timer
from the DT results in a booting machine?

Florian, can you please check whether PPI12 is actually the EL2
virtual timer on the RPI5 SoC?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.