Message-ID: <4768BB43.1000609@qumranet.com>
Date: Wed, 19 Dec 2007 08:33:39 +0200
From: Avi Kivity <avi@qumranet.com>
User-Agent: Thunderbird 2.0.0.9 (X11/20071115)
MIME-Version: 1.0
To: Ingo Molnar <mingo@elte.hu>
CC: Thomas Gleixner <tglx@linutronix.de>,
       kvm-devel <kvm-devel@lists.sourceforge.net>,
       linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: Guest kernel hangs in smp kvm for older kernels prior to tsc
 sync cleanup
References: <47680173.6060606@qumranet.com> <20071218221930.GA26109@elte.hu>
In-Reply-To: <20071218221930.GA26109@elte.hu>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1309
Lines: 34

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/