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[Qemu-devel] [Bug 1195012] Re: x86_64 and i386 return 0 when reading MSR
From: |
Jeff Rizzo |
Subject: |
[Qemu-devel] [Bug 1195012] Re: x86_64 and i386 return 0 when reading MSR_TSC |
Date: |
Wed, 26 Jun 2013 21:15:30 -0000 |
The NetBSD problem report where this issue was raised:
http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=47967
a workaround has been put in place for now, but it would be good to fix
this in QEMU.
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https://bugs.launchpad.net/bugs/1195012
Title:
x86_64 and i386 return 0 when reading MSR_TSC
Status in QEMU:
New
Bug description:
Running NetBSD 6.1 (i386 and amd64) under QEMU (from git - 1.5.50 is the
version it shows) results in an incorrectly set
TSC frequency (set to 0), because NetBSD uses rdmsr(TSC_MSR) for its
serializing CPU counter.
To reproduce the problem, you can run an install ISO of NetBSD 6.1
(either i386 or amd64, depending on which qemu). Quit out of the
installer, and you're left at a root prompt:
# sysctl machdep.tsc_freq
machdep.tsc_freq = 0
...on real hardware, it will return the TSC frequency:
# sysctl machdep.tsc_freq
machdep.tsc_freq = 3292685070
...this causes problems with a number of applications.
The NetBSD code which reads the MSR is here:
http://nxr.netbsd.org/xref/src/sys/arch/x86/x86/tsc.c#262
... the "rdmsr(MSR_TSC)" call in cpu_counter_serializing() always
returns 0 when run under QEMU.
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