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29cc0f3aa7
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: Hyesoo Yu <hyesoo.yu@samsung.com>
Reported-by: Quentin Perret <qperret@google.com>
Reviewed-by: Quentin Perret <qperret@google.com>
Fixes: 7d26b0516a ("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
Signed-off-by: Will Deacon <will@kernel.org>
81 lines
2.0 KiB
C
81 lines
2.0 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* Delay loops based on the OpenRISC implementation.
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*
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* Copyright (C) 2012 ARM Limited
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*
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* Author: Will Deacon <will.deacon@arm.com>
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*/
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#include <linux/delay.h>
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#include <linux/init.h>
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/timex.h>
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#include <clocksource/arm_arch_timer.h>
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#define USECS_TO_CYCLES(time_usecs) \
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xloops_to_cycles((time_usecs) * 0x10C7UL)
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static inline unsigned long xloops_to_cycles(unsigned long xloops)
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{
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return (xloops * loops_per_jiffy * HZ) >> 32;
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}
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/*
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* Force the use of CNTVCT_EL0 in order to have the same base as WFxT.
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* This avoids some annoying issues when CNTVOFF_EL2 is not reset 0 on a
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* KVM host running at EL1 until we do a vcpu_put() on the vcpu. When
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* running at EL2, the effective offset is always 0.
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*
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* Note that userspace cannot change the offset behind our back either,
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* as the vcpu mutex is held as long as KVM_RUN is in progress.
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*/
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#define __delay_cycles() __arch_counter_get_cntvct_stable()
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void __delay(unsigned long cycles)
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{
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cycles_t start = __delay_cycles();
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if (alternative_has_cap_unlikely(ARM64_HAS_WFXT)) {
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u64 end = start + cycles;
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/*
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* Start with WFIT. If an interrupt makes us resume
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* early, use a WFET loop to complete the delay.
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*/
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wfit(end);
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while ((__delay_cycles() - start) < cycles)
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wfet(end);
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} else if (arch_timer_evtstrm_available()) {
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const cycles_t timer_evt_period =
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USECS_TO_CYCLES(ARCH_TIMER_EVT_STREAM_PERIOD_US);
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while ((__delay_cycles() - start + timer_evt_period) < cycles)
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wfe();
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}
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while ((__delay_cycles() - start) < cycles)
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cpu_relax();
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}
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EXPORT_SYMBOL(__delay);
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inline void __const_udelay(unsigned long xloops)
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{
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__delay(xloops_to_cycles(xloops));
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}
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EXPORT_SYMBOL(__const_udelay);
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void __udelay(unsigned long usecs)
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{
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__const_udelay(usecs * 0x10C7UL); /* 2**32 / 1000000 (rounded up) */
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}
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EXPORT_SYMBOL(__udelay);
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void __ndelay(unsigned long nsecs)
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{
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__const_udelay(nsecs * 0x5UL); /* 2**32 / 1000000000 (rounded up) */
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}
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EXPORT_SYMBOL(__ndelay);
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