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"),

Sorry, I don't follow what you mean here.  Can you elaborate?

>   timer reinjection
> probably.

Timer reinjection will continue to work as expected.  If a guest is 
halting an external interrupt is delivered (by a timer), the guest will 
still exit as expected.

I can think of anything that would be functionally correct and still 
depend on getting hlt exits because ultimately, a guest never actually 
has to do a hlt (and certainly there are guests that won't).

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

If the desire is the ultimate desire is to have the guests be scheduled 
in a non-work conserving fashion, I can't see a more direct approach 
that to simply not have the guests yield (which is ultimately what hlt 
trapping does).

Anything the scheduler would do is after the fact and probably based on 
inference about why the yield.

Regards,

Anthony Liguori

>
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