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

[Daniel Drake]

On Fri, Aug 9, 2019 at 8:54 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.
>
> 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.

This solves the problem I described in the thread:
    setup_boot_APIC_clock() NULL dereference during early boot on
reduced hardware platforms

Thanks for your quick support!

> 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.

I can do a bit of testing on other platforms too. Are there any
specific tests I should run, other than checking that the system boots
and doesn't have any timer watchdog complaints in the log?

Thanks
Daniel