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Date: Mon, 14 Dec 2009 22:10:49 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] cpuid problem in upstream qemu with kvm
Message-ID: <20091214201049.GD6150@redhat.com>
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: qemu-devel@nongnu.org, Gleb Natapov <gleb@redhat.com>, avi@redhat.com

On Mon, Dec 14, 2009 at 02:02:03PM -0600, Anthony Liguori wrote:
> Gleb Natapov wrote:
>>> I thought KVM emulates the syscall instruction?  I swear I've seen
>>> patches for that.
>>>
>>>     
>> It is. Starting from 2.6.32.
>>   
>
> Okay, so this is a performance vs. migration compatibility issue then?
>
> BTW, couldn't we just not advertise syscall in cpuid?  That should fix  
> it too without sacrificing migration compatibility.  We get a slight  
> slowdown on AMD hosts but that's probably minor compared to the cost of  
> using emulated syscall on Intel hosts.
>
> Regards,
>
> Anthony Liguori

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.


-- 
MST