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From:   Daniel Drake <drake@endlessm.com>
Date:   Mon, 12 Aug 2019 14:16:02 +0800
Message-ID: <CAD8Lp46FgT6yoW9a4Yt8t=bVWzZbYHjw-Dqdk6Pvd2xzxfGHLQ@mail.gmail.com>
Subject: Re: [PATCH] x86/apic: Handle missing global clockevent gracefully
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
        "Li, Aubrey" <aubrey.li@linux.intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Endless Linux Upstreaming Team <linux@endlessm.com>,
        Jiri Slaby <jslaby@suse.cz>
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On Fri, Aug 9, 2019 at 8:54 PM Thomas Gleixner <tglx@linutronix.de> 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