Re: [PATCH 3/9] change order of kvm_init call.

[Jan Kiszka <jan.kiszka@web.de>]

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Glauber Costa wrote:
> On Mon, Jul 27, 2009 at 12:49:06PM -0500, Anthony Liguori wrote:
>> Glauber Costa wrote:
>>> On Sun, Jul 26, 2009 at 08:59:44PM +0200, Jan Kiszka wrote:
>>>   
>>>> Glauber Costa wrote:
>>>>     
>>>>> The goal is to get rid of the call to kvm_init. But those things
>>>>> are subtle, and often break. So do it in a separate patch, to help
>>>>> finding potential issues in future bisections.
>>>>>       
>>>> Found such an issued: This patch triggers a segfault if no kvm modules
>>>> are loaded and you start qemu without -no-kvm. Please have a look.
>>>>
>>>> Jan
>>>>     
>>> ok, the culprit seems to be a 
>>>
>>>     if (kvm_enabled())
>>>         return;
>>>
>>> in the beginning of code_gen_alloc.
>>>
>>> It is 7f3d0cbe, by Avi, and according to changelog, suggested by anthony.
>>> I however, fail to realise the purpose of this optimization. For one thing,
>>> it totally dictates that kvm has absolutely to be enabled or disabled prior
>>> to this point. No mind changing later. Also, the real deal is to be able
>>> to compile out tcg entirely. The strategy of just disabling the code gen
>>> alloc is a minor nitpick that just papers over this.
>>>   
>> I agree with you in principle but I think reverting this papers over an  
>> issue.
>>
>> Why are we touching code_gen_ptr when using KVM?  Can someone post the  
>> full back trace?
> we're not.
> 
> The issue happens exactly when the kvm modules are not loaded, then we're failing
> to initialize kvm. However, in the patch that raised this issue, I'm moving
> KVM initialization to after this code path. And in qemu-kvm.git, kvm is
> enabled-by-default. So tcg code would think kvm is enabled and skip initialization,
> while kvm code will fail to really initialize itself later.
> 
> Result? Mayhem.
> 

I think we should simply resolves this the way upstream does: Do not
start if modules are missing and -no-kvm is omitted - or even switch
over to -enable-kvm as I think you already suggested in some other
thread. Then we can either fail or succeed, but not fall back more or
less silently. This falling back of qemu-kvm to tcg is a constant source
of confusion anyway.

Jan


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