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SubjectRe: [patch 1/8] rtc: mc146818: Prevent reading garbage - bug
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Hi,

After some bisecting, I found that commit 05a0302c3548 ("rtc: mc146818:
Prevent reading garbage", this patch, introduced since v5.11-rc1) makes
my VM hang at boot. Before this commit, I got this (and didn't notice)
at every boot:
rtc_cmos rtc_cmos: registered as rtc0
rtc_cmos rtc_cmos: hctosys: unable to read the hardware clock
rtc_cmos rtc_cmos: alarms up to one day, 114 bytes nvram

I notice that this patch creates infinite loops, which my VM falls into
(cf. below).

I didn't succeed to properly fix this without a revert. I tried to set a
maximum number of jumps, but I got pvqspinlock warnings.

Regards,
Mickaël


On 06/12/2020 22:46, Thomas Gleixner wrote:
> The MC146818 driver is prone to read garbage from the RTC. There are
> several issues all related to the update cycle of the MC146818. The chip
> increments seconds obviously once per second and indicates that by a bit in
> a register. The bit goes high 244us before the actual update starts. During
> the update the readout of the time values is undefined.
>
> The code just checks whether the update in progress bit (UIP) is set before
> reading the clock. If it's set it waits arbitrary 20ms before retrying,
> which is ample because the maximum update time is ~2ms.
>
> But this check does not guarantee that the UIP bit goes high and the actual
> update happens during the readout. So the following can happen
>
> 0.997 UIP = False
> -> Interrupt/NMI/preemption
> 0.998 UIP -> True
> 0.999 Readout <- Undefined
>
> To prevent this rework the code so it checks UIP before and after the
> readout and if set after the readout try again.
>
> But that's not enough to cover the following:
>
> 0.997 UIP = False
> Readout seconds
> -> NMI (or vCPU scheduled out)
> 0.998 UIP -> True
> update completes
> UIP -> False
> 1.000 Readout minutes,....
> UIP check succeeds
>
> That can make the readout wrong up to 59 seconds.
>
> To prevent this, read the seconds value before the first UIP check,
> validate it after checking UIP and after reading out the rest.
>
> It's amazing that the original i386 code had this actually correct and
> the generic implementation of the MC146818 driver got it wrong in 2002 and
> it stayed that way until today.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> drivers/rtc/rtc-mc146818-lib.c | 64 ++++++++++++++++++++++++-----------------
> 1 file changed, 39 insertions(+), 25 deletions(-)
>
> --- a/drivers/rtc/rtc-mc146818-lib.c
> +++ b/drivers/rtc/rtc-mc146818-lib.c
> @@ -8,41 +8,41 @@
> #include <linux/acpi.h>
> #endif
>
> -/*
> - * Returns true if a clock update is in progress
> - */
> -static inline unsigned char mc146818_is_updating(void)
> -{
> - unsigned char uip;
> - unsigned long flags;
> -
> - spin_lock_irqsave(&rtc_lock, flags);
> - uip = (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP);
> - spin_unlock_irqrestore(&rtc_lock, flags);
> - return uip;
> -}
> -
> unsigned int mc146818_get_time(struct rtc_time *time)
> {
> unsigned char ctrl;
> unsigned long flags;
> unsigned char century = 0;
> + bool retry;
>
> #ifdef CONFIG_MACH_DECSTATION
> unsigned int real_year;
> #endif
>
> +again:
> + spin_lock_irqsave(&rtc_lock, flags);
> /*
> - * read RTC once any update in progress is done. The update
> - * can take just over 2ms. We wait 20ms. There is no need to
> - * to poll-wait (up to 1s - eeccch) for the falling edge of RTC_UIP.
> - * If you need to know *exactly* when a second has started, enable
> - * periodic update complete interrupts, (via ioctl) and then
> - * immediately read /dev/rtc which will block until you get the IRQ.
> - * Once the read clears, read the RTC time (again via ioctl). Easy.
> + * Check whether there is an update in progress during which the
> + * readout is unspecified. The maximum update time is ~2ms. Poll
> + * every msec for completion.
> + *
> + * Store the second value before checking UIP so a long lasting NMI
> + * which happens to hit after the UIP check cannot make an update
> + * cycle invisible.
> */
> - if (mc146818_is_updating())
> - mdelay(20);
> + time->tm_sec = CMOS_READ(RTC_SECONDS);
> +
> + if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) {
> + spin_unlock_irqrestore(&rtc_lock, flags);
> + mdelay(1);

My VM loops here.
time->tm_sec is always 255.

> + goto again;
> + }
> +
> + /* Revalidate the above readout */
> + if (time->tm_sec != CMOS_READ(RTC_SECONDS)) {
> + spin_unlock_irqrestore(&rtc_lock, flags);
> + goto again;
> + }
>
> /*
> * Only the values that we read from the RTC are set. We leave
> @@ -50,8 +50,6 @@ unsigned int mc146818_get_time(struct rt
> * RTC has RTC_DAY_OF_WEEK, we ignore it, as it is only updated
> * by the RTC when initially set to a non-zero value.
> */
> - spin_lock_irqsave(&rtc_lock, flags);
> - time->tm_sec = CMOS_READ(RTC_SECONDS);
> time->tm_min = CMOS_READ(RTC_MINUTES);
> time->tm_hour = CMOS_READ(RTC_HOURS);
> time->tm_mday = CMOS_READ(RTC_DAY_OF_MONTH);
> @@ -66,8 +64,24 @@ unsigned int mc146818_get_time(struct rt
> century = CMOS_READ(acpi_gbl_FADT.century);
> #endif
> ctrl = CMOS_READ(RTC_CONTROL);
> + /*
> + * Check for the UIP bit again. If it is set now then
> + * the above values may contain garbage.
> + */
> + retry = CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP;
> + /*
> + * A NMI might have interrupted the above sequence so check whether
> + * the seconds value has changed which indicates that the NMI took
> + * longer than the UIP bit was set. Unlikely, but possible and
> + * there is also virt...
> + */
> + retry |= time->tm_sec != CMOS_READ(RTC_SECONDS);
> +
> spin_unlock_irqrestore(&rtc_lock, flags);
>
> + if (retry)
> + goto again;
> +
> if (!(ctrl & RTC_DM_BINARY) || RTC_ALWAYS_BCD)
> {
> time->tm_sec = bcd2bin(time->tm_sec);
>
>

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