Re: [PATCH 4/5] KVM: SVM: Allow EFER.LMSLE to be set with nested svm

[Joerg Roedel <joro@8bytes.org>]

On Wed, May 05, 2010 at 05:46:59PM +0300, Avi Kivity wrote:
> On 05/05/2010 05:04 PM, Joerg Roedel wrote:
>> This patch enables setting of efer bit 13 which is allowed
>> in all SVM capable processors. This is necessary for the
>> SLES11 version of Xen 4.0 to boot with nested svm.
>>    
>
> Interesting, why does it require it?

I don't know. I traced the Xen crash down and found that is gets a #GP
because it tries to set this bit.

> Obviously it isn't needed since it manages to run on Intel without it.

I have heard inofficial statements that they set this bit to provide the
functionality to their guests. And Xen sets this bit together with the
SVM bit.

>>   /* Intel MSRs. Some also available on other CPUs */
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index 74f7b9d..bc087c7 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -610,7 +610,7 @@ static __init int svm_hardware_setup(void)
>>
>>   	if (nested) {
>>   		printk(KERN_INFO "kvm: Nested Virtualization enabled\n");
>> -		kvm_enable_efer_bits(EFER_SVME);
>> +		kvm_enable_efer_bits(EFER_SVME | EFER_LMSLE);
>>   	}
>>
>>   	for_each_possible_cpu(cpu) {
>>    
>
> What if the host doesn't have it?

It is present in all SVM capable AMD processors.

> Why enable it only for the nested case?  It's not svm specific (it's  
> useful for running non-hvm Xen in non-nested mode).

Because there is no cpuid bit for this feature. You can roughly check
for it using the svm cpuid bit.


	Joerg