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Date: Mon, 14 Dec 2009 15:49:39 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] cpuid problem in upstream qemu with kvm
References: <20091214193541.GA6150@redhat.com> <4B269596.1050103@codemonkey.ws>
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: qemu-devel@nongnu.org, Gleb Natapov <gleb@redhat.com>, avi@redhat.com

Michael S. Tsirkin wrote:
> On Mon, Dec 14, 2009 at 02:54:49PM -0600, Anthony Liguori wrote:
>   
>> Michael S. Tsirkin wrote:
>>     
>>> On Mon, Dec 14, 2009 at 02:18:33PM -0600, Anthony Liguori wrote:
>>>   
>>>       
>>>> Michael S. Tsirkin wrote:
>>>>     
>>>>         
>>>>> This might help 32 bit guests, but not guests with 64 bit
>>>>> kernel and 32 bit userspace (my case) because all 64 bit
>>>>> CPUs advertise syscall bit in cpuid. Thus 64 bit guests
>>>>> do not seem to even bother checking this bit:
>>>>> AMD + 64 bit -> syscall.
>>>>>         
>>>>>           
>>>> Okay, I don't see a great option other than migrating the vendor_id string.
>>>>     
>>>>         
>>> This won't help with kernels <2.6.32 though.  I guess we can switch
>>> default vendor to Intel, this likely has some other side effects.
>>>   
>>>       
>> That's a kernel bug.  If we think it effects a lot of users, we should  
>> introduce a CAP such that we can detect this in userspace and fail  
>> gracefully.
>>     
>
> Not emulating feature host CPU does not have is a kernel bug?
> Okay ...
> Yes, almost no one runs 2.6.32 yet.
>   

The kernel has the ability to filter feature bits from cpuid.  We assume 
it's going to filter out things it doesn't support.

Regards,

Anthony Liguori