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Date: Fri, 2 Nov 2001 14:04:16 -0800 (PST)
From: Christian Ruppert <arc@gmx.li>
To: freebsd-gnats-submit@FreeBSD.org
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Subject: i386/31716: FreeBSD uses broken tsc timecounter by default
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>Number:         31716
>Category:       i386
>Synopsis:       FreeBSD uses broken tsc timecounter by default
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Fri Nov 02 14:10:01 PST 2001
>Closed-Date:
>Last-Modified:
>Originator:     Christian Ruppert
>Release:        FreeBSD4.3, security patches 01:42, 01:52, 01:53, 01:55 installed
>Organization:
-
>Environment:
FreeBSD arc.rezel.enst.fr 4.3-RELEASE FreeBSD 4.3-RELEASE #1: Fri Nov  2 21:15:39 CET 2001     root@arc.rezel.enst.fr:/usr/src/sys/compile/nevermind  i386
>Description:
although the kernel detects the cpu (AMD K5 model 0) correctly and thus (should) set tsc_is_broken = 1 in line 282 of i386/i386/identcpu.c it uses tsc as default timecounter. this results in a disfunctional system clock (since tsc is really broken at least on my k5 model 0) until you change the method using sysctl -w kern.timecounter.hardware=i8254.
>How-To-Repeat:
boot the kernel (without apm or smp support compiled in) on an AMD K5 model 0 machine and take a look at sysctl kern.timecounter.hardware.
i expect similar behaviour using an IDT WinChip C6 since FreeBSD uses the same mechanism here (setting tsc_is_bromen = 1) however i could not verify this.
>Fix:
WORKAROUND: sysctl -w kern.timecounter.hardware=i8254 after booting.
however this results in an offset (usually some seconds) of the system clock due to the period the broken timecounter was used.
>Release-Note:
>Audit-Trail:
>Unformatted:

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