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

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Message-ID: <b7c3809f-cdea-03f0-09f8-a47e6f304ec1@suse.cz>
Date:   Fri, 9 Aug 2019 14:59:04 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>
Cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Daniel Drake <drake@...lessm.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Endless Linux Upstreaming Team <linux@...lessm.com>
Subject: Re: [PATCH] x86/apic: Handle missing global clockevent gracefully

On 09. 08. 19, 14:54, 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.
> 
> 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@...lessm.com>
> Reported-by: Jiri Slaby <jslaby@...e.cz>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> 
> Note: Only lightly tested, but of course not on an affected machine.

Thanks, I will make them test the patch and let you know.

-- 
js
suse labs