Re: Broken udelay() on KVM host with a vcpu loaded

Quentin Perret
2026-02-10T15:58:19Z

On Tuesday 10 Feb 2026 at 15:34:22 (+0000), Will Deacon wrote:
> On Tue, Feb 10, 2026 at 12:52:21PM +0000, Marc Zyngier wrote:
> > On Tue, 10 Feb 2026 12:27:48 +0000,
> > Quentin Perret <qperret@google.com> wrote:
> > >
> > > Hi all,
> > >
> > > I have just received a report from a partner of udelay misbehaving when
> > > running on the host whilst a vCPU is loaded. This hardware has FEAT_WFxT
> > > and uses the matching implementation of udelay. Interestingly, WFIT
> > > triggers using CNTVCT_EL0 unconditionally, but with KVM the host/guest
> > > switch for that happens from the preempt notifiers/vcpu_put which aren't
> > > invoked when e.g. handling an IRQ. Interestingly, udelay reads the arch
> > > timer to set the waiting time for WFIT using an absolute value, and that
> > > gets compared to CNTVCT_EL0 which in the aforementioned
> > > IRQ-with-vCPU-loaded case uses the _guest's_ CNTVCT_EL0.
> >
> > Well, the underlying issue is that get_cycle(), as used by __delay(),
> > is *either* using CNTVCT_EL0 (when booted at EL1) or CNTPCT_EL0 (when
> > booted at EL2).
> >
> > >
> > > I can think of two approaches to address the problem:
> > > 1. have KVM context switch cntvoff proactively prior to re-enabling
> > > preemption when handling a guest exit;
> > > 2. modify the WFIT-based udelay implementation to read from CNTVCT_EL0
> > > instead of the arch_timer to be a bit more self-consitent;
> > >
> > > Other ideas welcome!
>
> I suppose a third option would be to avoid WFxT when we're on a CPU
> with a vCPU loaded on it? The per-cpu state is grotty, mind.

It's one of the rare cases where we could look at that per-cpu data in
preemptible context, because that per-cpu state would migrate with the
task if it were migrated to a different CPU thanks to the preempt
notifiers, so that's nice.

> A bigger hammer would be to use WFxT only when using VHE.
>
> > (1) is a real nightmare, and would force a complete redesign of the
> > life cycle of guest timers (switching from load/put to enter/exit for
> > the context switch, but only on !VHE). I'd rather avoid that, as this
> > is a pretty large performance penalty.
> >
> > (2) is much more palatable, and easily hacked, see below. Can you
> > please five it a go?
> >
> > Thanks,
> >
> > M.
> >
> > From b1b45d591aed3e5276ff857dbc6cfa3bce181766 Mon Sep 17 00:00:00 2001
> > From: Marc Zyngier <maz@kernel.org>
> > Date: Tue, 10 Feb 2026 12:43:07 +0000
> > Subject: [PATCH] arm64: Force the use of CNTVCT_EL0 in __delay()
> >
> > Quentin reports an interesting problem with the use of WFxT in __delay()
> > when a vcpu is loaded and that KVM is *not* in VHE mode.
> >
> > In this case, CNTVOFF_EL2 is set to a non-zero value to reflect the
> > state of the guest virtual counter. At the same time, __delay() is
> > using get_cycles() to read the counter value, which is indirected to
> > reading CNTPCT_EL0.
> >
> > The core of the issue is that WFxT is using the *virtual* counter,
> > while the kernel is using the physical counter, and that the offset
> > introduces a really bad discrepancy between the two.
> >
> > Fix this by forcing the use of CNTVCT_EL0, making __delay() consistent
> > irrespective of the value of CNTVOFF_EL2.
> >
> > Reported-by: Quentin Perret <qperret@google.com>
> > Fixes: 7d26b0516a0df ("arm64: Use WFxT for __delay() when possible")
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > Link: https://lore.kernel.org/r/ktosachvft2cgqd5qkukn275ugmhy6xrhxur4zqpdxlfr3qh5h@o3zrfnsq63od
> > Cc: stable@vger.kernel.org
> > ---
> > arch/arm64/lib/delay.c | 15 +++++++++++----
> > 1 file changed, 11 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/arm64/lib/delay.c b/arch/arm64/lib/delay.c
> > index cb2062e7e2340..26a39bb301ef6 100644
> > --- a/arch/arm64/lib/delay.c
> > +++ b/arch/arm64/lib/delay.c
> > @@ -23,9 +23,16 @@ static inline unsigned long xloops_to_cycles(unsigned long xloops)
> > return (xloops * loops_per_jiffy * HZ) >> 32;
> > }
> >
> > +/*
> > + * Force the use of CNTVCT_EL0 in order to have the same base as
> > + * WFxT. This avoids some annoying issues when CNTVOFF_EL2 is not
> > + * reset 0 on a KVM host until we do a vcpu_put() on the vcpu.
> > + */
> > +#define __delay_cycles() __arch_counter_get_cntvct_stable()
> > +
> > void __delay(unsigned long cycles)
> > {
> > - cycles_t start = get_cycles();
> > + cycles_t start = __delay_cycles();
> >
> > if (alternative_has_cap_unlikely(ARM64_HAS_WFXT)) {
> > u64 end = start + cycles;
> > @@ -35,17 +42,17 @@ void __delay(unsigned long cycles)
> > * early, use a WFET loop to complete the delay.
> > */
> > wfit(end);
> > - while ((get_cycles() - start) < cycles)
> > + while ((__delay_cycles() - start) < cycles)
> > wfet(end);
> > } else if (arch_timer_evtstrm_available()) {
> > const cycles_t timer_evt_period =
> > USECS_TO_CYCLES(ARCH_TIMER_EVT_STREAM_PERIOD_US);
> >
> > - while ((get_cycles() - start + timer_evt_period) < cycles)
> > + while ((__delay_cycles() - start + timer_evt_period) < cycles)
> > wfe();
> > }
> >
> > - while ((get_cycles() - start) < cycles)
> > + while ((__delay_cycles() - start) < cycles)
> > cpu_relax();
>
> I can't put my finger on a specific bug, but it does feel pretty scary
> to use the virtual counter in the host while running with a guest
> voffset. Is the offset userspace controllable? If so, can we guarantee
> that we're ok with overflow and is it ok if we get preempted in the
> middle of the loop? It's making my head hurt!

Ouch, it does seem that the SET_ONE_REG stuff allows to mess with that
value _out of vcpu context_, so yeah userspace could change the value
while a vcpu thread is preempted in the middle of a udelay loop ...

> The smp_cond_load_{relaxed,acquire}_timeout() series from Ankur looks
> like it has the same issue (it uses arch_timer_read_counter()).
>
> Will