Re: [PATCH] kvm-vmx: add module parameter to avoid trapping HLT instructions (v2)

[Anthony Liguori <anthony@codemonkey.ws>]

On 12/02/2010 11:37 AM, Marcelo Tosatti wrote:
> On Thu, Dec 02, 2010 at 07:59:17AM -0600, Anthony Liguori wrote:
>    
>> In certain use-cases, we want to allocate guests fixed time slices where idle
>> guest cycles leave the machine idling.  There are many approaches to achieve
>> this but the most direct is to simply avoid trapping the HLT instruction which
>> lets the guest directly execute the instruction putting the processor to sleep.
>>
>> Introduce this as a module-level option for kvm-vmx.ko since if you do this
>> for one guest, you probably want to do it for all.  A similar option is possible
>> for AMD but I don't have easy access to AMD test hardware.
>>
>> Signed-off-by: Anthony Liguori<aliguori@us.ibm.com>
>> ---
>> v1 ->  v2
>>   - Rename parameter to yield_on_hlt
>>   - Remove __read_mostly
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index caa967e..d8310e4 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -69,6 +69,9 @@ module_param(emulate_invalid_guest_state, bool, S_IRUGO);
>>   static int __read_mostly vmm_exclusive = 1;
>>   module_param(vmm_exclusive, bool, S_IRUGO);
>>
>> +static int yield_on_hlt = 1;
>> +module_param(yield_on_hlt, bool, S_IRUGO);
>> +
>>   #define KVM_GUEST_CR0_MASK_UNRESTRICTED_GUEST				\
>>   	(X86_CR0_WP | X86_CR0_NE | X86_CR0_NW | X86_CR0_CD)
>>   #define KVM_GUEST_CR0_MASK						\
>> @@ -1419,7 +1422,7 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
>>   				&_pin_based_exec_control)<  0)
>>   		return -EIO;
>>
>> -	min = CPU_BASED_HLT_EXITING |
>> +	min =
>>   #ifdef CONFIG_X86_64
>>   	      CPU_BASED_CR8_LOAD_EXITING |
>>   	      CPU_BASED_CR8_STORE_EXITING |
>> @@ -1432,6 +1435,10 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
>>   	      CPU_BASED_MWAIT_EXITING |
>>   	      CPU_BASED_MONITOR_EXITING |
>>   	      CPU_BASED_INVLPG_EXITING;
>> +
>> +	if (yield_on_hlt)
>> +		min |= CPU_BASED_HLT_EXITING;
>> +
>>   	opt = CPU_BASED_TPR_SHADOW |
>>   	      CPU_BASED_USE_MSR_BITMAPS |
>>   	      CPU_BASED_ACTIVATE_SECONDARY_CONTROLS;
>> -- 
>> 1.7.0.4
>>      
> Breaks async PF (see "checks on guest state"), timer reinjection
> probably.

In v3, I set the activity state to ACTIVE if the state is currently HLT 
when injecting an exception into a guest.

The effect is that after the exception is handled, if iret is executed, 
the hlt instruction will be restarted.  The seems like the correct 
semantics to me.

Regards,

Anthony Liguori

>   It should be possible to achieve determinism with
> a scheduler policy?
>
>
>
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