Re: [PATCH] x86/apic: Handle missing global clockevent gracefully

From: Li, Aubrey
Date: Mon Aug 12 2019 - 03:46:55 EST


On 2019/8/9 20:54, Thomas Gleixner wrote:
> Some newer machines do not advertise legacy timers. The kernel can handle
> that situation if the TSC and the CPU frequency are enumerated by CPUID or
> MSRs and the CPU supports TSC deadline timer. If the CPU does not support
> TSC deadline timer the local APIC timer frequency has to be known as well.
>
> Some Ryzens machines do not advertize legacy timers, but there is no
> reliable way to determine the bus frequency which feeds the local APIC
> timer when the machine allows overclocking of that frequency.

Are these platforms are all ACPI HW-reduced platform?

>
> As there is no legacy timer the local APIC timer calibration crashes due to
> a NULL pointer dereference when accessing the not installed global clock
> event device.
>
> Switch the calibration loop to a non interrupt based one, which polls
> either TSC (frequency known) or jiffies. The latter requires a global
> clockevent. As the machines which do not have a global clockevent installed
> have a known TSC frequency this is a non issue. For older machines where
> TSC frequency is not known, there is no known case where the legacy timers
> do not exist as that would have been reported long ago.
>
> Reported-by: Daniel Drake <drake@xxxxxxxxxxxx>
> Reported-by: Jiri Slaby <jslaby@xxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>
> Note: Only lightly tested, but of course not on an affected machine.
>
> If that works reliably, then this needs some exhaustive testing
> on a wide range of systems so we can risk backports to stable
> kernels.
>
> ---
> arch/x86/kernel/apic/apic.c | 70 +++++++++++++++++++++++++++++++++-----------
> include/linux/acpi_pmtmr.h | 10 ++++++
> 2 files changed, 64 insertions(+), 16 deletions(-)
>
> --- a/arch/x86/kernel/apic/apic.c
> +++ b/arch/x86/kernel/apic/apic.c
> @@ -851,7 +851,8 @@ bool __init apic_needs_pit(void)
> static int __init calibrate_APIC_clock(void)
> {
> struct clock_event_device *levt = this_cpu_ptr(&lapic_events);
> - void (*real_handler)(struct clock_event_device *dev);
> + u64 tsc_perj = 0, tsc_start = 0;
> + unsigned long jif_start;
> unsigned long deltaj;
> long delta, deltatsc;
> int pm_referenced = 0;
> @@ -878,29 +879,65 @@ static int __init calibrate_APIC_clock(v
> apic_printk(APIC_VERBOSE, "Using local APIC timer interrupts.\n"
> "calibrating APIC timer ...\n");
>
> - local_irq_disable();
> -
> - /* Replace the global interrupt handler */
> - real_handler = global_clock_event->event_handler;
> - global_clock_event->event_handler = lapic_cal_handler;
> + /*
> + * There are platforms w/o global clockevent devices. Instead of
> + * making the calibration conditional on that, use a polling based
> + * approach everywhere.
> + */
>
> + local_irq_disable();
> /*
> * Setup the APIC counter to maximum. There is no way the lapic
> * can underflow in the 100ms detection time frame
> */
> __setup_APIC_LVTT(0xffffffff, 0, 0);
>
> - /* Let the interrupts run */
> - local_irq_enable();
> + /*
> + * Methods to terminate the calibration loop:
> + * 1) Global clockevent if available (jiffies)
> + * 2) TSC if available and frequency is known
> + */
> + jif_start = READ_ONCE(jiffies);
> +
> + if (tsc_khz) {
> + tsc_start = rdtsc();
> + tsc_perj = div_u64((u64)tsc_khz * 1000, HZ);
> + }
> +
> + while (lapic_cal_loops <= LAPIC_CAL_LOOPS) {

Is this loop still meaningful, can we just invoke the handler twice
before and after the tick?

Thanks,
-Aubrey

> + /*
> + * Enable interrupts so the tick can fire, if a global
> + * clockevent device is available
> + */
> + local_irq_enable();
>
> - while (lapic_cal_loops <= LAPIC_CAL_LOOPS)
> - cpu_relax();
> + /* Wait for a tick to elapse */
> + while (1) {
> + if (tsc_khz) {
> + u64 tsc_now = rdtsc();
> + if ((tsc_now - tsc_start) >= tsc_perj) {
> + tsc_start += tsc_perj;
> + break;
> + }
> + } else {
> + unsigned long jif_now = READ_ONCE(jiffies);
> +
> + if (time_after(jif_now, jif_start)) {
> + jif_start = jif_now;
> + break;
> + }
> + }
> + cpu_relax();
> + }
> +
> + /* Invoke the calibration routine */
> + local_irq_disable();
> + lapic_cal_handler(NULL);
> + local_irq_enable();
> + }
>
> local_irq_disable();
>
> - /* Restore the real event handler */
> - global_clock_event->event_handler = real_handler;
> -
> /* Build delta t1-t2 as apic timer counts down */
> delta = lapic_cal_t1 - lapic_cal_t2;
> apic_printk(APIC_VERBOSE, "... lapic delta = %ld\n", delta);
> @@ -943,10 +980,11 @@ static int __init calibrate_APIC_clock(v
> levt->features &= ~CLOCK_EVT_FEAT_DUMMY;
>
> /*
> - * PM timer calibration failed or not turned on
> - * so lets try APIC timer based calibration
> + * PM timer calibration failed or not turned on so lets try APIC
> + * timer based calibration, if a global clockevent device is
> + * available.
> */
> - if (!pm_referenced) {
> + if (!pm_referenced && global_clock_event) {
> apic_printk(APIC_VERBOSE, "... verify APIC timer\n");
>
> /*
> --- a/include/linux/acpi_pmtmr.h
> +++ b/include/linux/acpi_pmtmr.h
> @@ -18,6 +18,11 @@
> extern u32 acpi_pm_read_verified(void);
> extern u32 pmtmr_ioport;
>
> +static inline bool acpi_pm_timer_available(void)
> +{
> + return pmtmr_ioport != 0;
> +}
> +
> static inline u32 acpi_pm_read_early(void)
> {
> if (!pmtmr_ioport)
> @@ -28,6 +33,11 @@ static inline u32 acpi_pm_read_early(voi
>
> #else
>
> +static inline bool acpi_pm_timer_available(void)
> +{
> + return false;
> +}
> +
> static inline u32 acpi_pm_read_early(void)
> {
> return 0;
>