RE: [PATCH 2/2] Allow the SVM CPUID bit in a VM

["Itamar Heim" <itamarh@gmail.com>]

Userspace may know better, for example when you want to allow live migration
between multiple hosts and want to calculate lowest common denominator
between them.

-----Original Message-----
From: kvm-owner@vger.kernel.org [mailto:kvm-owner@vger.kernel.org] On Behalf
Of Alexander Graf
Sent: Tuesday, September 02, 2008 20:54 PM
To: Alexander Graf
Cc: Avi Kivity; kvm@vger.kernel.org; joro@8bytes.org; anthony@codemonkey.ws
Subject: Re: [PATCH 2/2] Allow the SVM CPUID bit in a VM


On Sep 2, 2008, at 7:42 PM, Alexander Graf wrote:

>
> On Sep 1, 2008, at 4:00 PM, Avi Kivity wrote:
>
>> Alexander Graf wrote:
>>> Usually the qemu-kvm-bridge removes the SVM capability flag. Since  
>>> KVM now
>>> supports nested SVM, this is no longer necessary.
>>>
>>> Signed-off-by: Alexander Graf <agraf@suse.de>
>>> ---
>>> qemu/qemu-kvm-x86.c |    3 ---
>>> 1 files changed, 0 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/qemu/qemu-kvm-x86.c b/qemu/qemu-kvm-x86.c
>>> index 5daedd1..cfc533f 100644
>>> --- a/qemu/qemu-kvm-x86.c
>>> +++ b/qemu/qemu-kvm-x86.c
>>> @@ -489,9 +489,6 @@ static void do_cpuid_ent(struct  
>>> kvm_cpuid_entry *e, uint32_t function,
>>> 	// nx
>>> 	if ((h_edx & 0x00100000) == 0)
>>> 	    e->edx &= ~0x00100000u;
>>> -	// svm
>>> -	if (e->ecx & 4)
>>> -	    e->ecx &= ~4u;
>>>    }
>>>
>>
>> Needs to be predicated on a KVM_CAP_ test, so we don't enable this  
>> on older kvms.
>
> Is this a real problem? We don't allow setting SVME on older kernels  
> anyways.

On a sidenote I'm really not fond of the userspace CPUID capability  
restrictions in the first place. Shouldn't this all be determined in  
the kernel module? It doesn't make since IMHO to restrict NX usage  
from userspace. The kernel module knows best what it's capable of.

Alex

>
>
> Alex
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