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

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Message-ID: <4768BB43.1000609@qumranet.com>
Date:	Wed, 19 Dec 2007 08:33:39 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Guest kernel hangs in smp kvm for older kernels prior to tsc
 sync cleanup

Ingo Molnar wrote:
>> 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?
>>     
>
> hm. Those time warps were really small, due to the small imperfections 
> in the "sync up all CPUs to the same moment and do a WRMSR to clear all 
> their TSCs" mechanism. I.e. at most a few usec time warps. I really dont 
> know how that should result in udevd hanging. Can you debug udevd in any 
> way?
>
>   

Adding debug didn't help.  I'll try some sysrq keys to see what the 
guest thinks is happening.

> so the only thing that KVM might be doing incorrectly here is the 
> emulation of the WRMSR that clears the TSC of each vcpu?
>   

By inspection, it is correct.  Of course I may be missing something, so 
I'll write a unit test for it.  It should also be much slower than the 
native wrmsr.

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

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