Commit 29cc0f3a authored by Marc Zyngier's avatar Marc Zyngier Committed by Will Deacon
Browse files

arm64: Force the use of CNTVCT_EL0 in __delay() 



Quentin forwards a report from Hyesoo Yu, describing an interesting
problem with the use of WFxT in __delay() when a vcpu is loaded and
that KVM is *not* in VHE mode (either nVHE or hVHE).

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: default avatarHyesoo Yu <hyesoo.yu@samsung.com>
Reported-by: default avatarQuentin Perret <qperret@google.com>
Reviewed-by: default avatarQuentin Perret <qperret@google.com>
Fixes: 7d26b051 ("arm64: Use WFxT for __delay() when possible")
Signed-off-by: default avatarMarc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/ktosachvft2cgqd5qkukn275ugmhy6xrhxur4zqpdxlfr3qh5h@o3zrfnsq63od


Cc: stable@vger.kernel.org
Signed-off-by: default avatarWill Deacon <will@kernel.org>
parent 36c0de02

arch/arm64/lib/delay.c

+15 −4
Original line number Diff line number Diff line
@@ -23,9 +23,20 @@ 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 running at EL1 until we do a vcpu_put() on the vcpu. When

 * running at EL2, the effective offset is always 0.

 *

 * Note that userspace cannot change the offset behind our back either,

 * as the vcpu mutex is held as long as KVM_RUN is in progress.

 */

#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 +46,17 @@ 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);