linux-kernel - Guest kernel hangs in smp kvm for older kernels prior to tsc sync cleanup

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Date:	Tue, 18 Dec 2007 19:20:51 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>
CC:	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Guest kernel hangs in smp kvm for older kernels prior to tsc sync
 cleanup

Booting RHEL 5 i386 in kvm with -no-kvm-irqchip -smp 4 will hang in 
udev.  I bisected this to a change in the _guest_ kernel:

> commit 95492e4646e5de8b43d9a7908d6177fb737b61f0
> Author: Ingo Molnar <mingo@...e.hu>
> Date:   Fri Feb 16 01:27:34 2007 -0800
>
>     [PATCH] x86: rewrite SMP TSC sync code
>
>     make the TSC synchronization code more robust, and unify it 
> between x86_64 and
>     i386.
>
>     The biggest change is the removal of the 'fix up TSCs' code on 
> x86_64 and
>     i386, in some rare cases it was /causing/ time-warps on SMP systems.
>
>     The new code only checks for TSC asynchronity - and if it can prove a
>     time-warp (if it can observe the TSC going backwards when going 
> from one CPU
>     to another within a critical section), then the TSC clock-source 
> is turned
>     off.
>
>     The TSC synchronization-checking code also got moved into a 
> separate file.

So, guest kernels prior to this commit will hang in kvm smp; after this 
commit they will boot fine.

While the change mentions that it fixes a time warp bug, it also says it 
should be rare.  So clearly kvm smp tsc handing is buggy.  Ingo/Thomas, 
(or anybody else), do you have any insight as to what kvm can be doing 
wrong to trigger this behavior?


-- 
error compiling committee.c: too many arguments to function

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