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

Marc Zyngier
2026-02-10T12:52:24Z

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!

(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();
}
EXPORT_SYMBOL(__delay);
--
2.47.3

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