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

[Anthony Liguori <anthony@codemonkey.ws>]

On 12/02/2010 08:39 AM, lidong chen wrote:
> In certain use-cases, we want to allocate guests fixed time slices where idle
> guest cycles leave the machine idling.
>
> i could not understand why need this? can you tell more detailedly?
>    

If you run 4 guests on a CPU, and they're all trying to consume 100% 
CPU, all things being equal, you'll get ~25% CPU for each guest.

However, if one guest is idle, you'll get something like 1% 32% 33% 
32%.  This characteristic is usually desirable because it increase 
aggregate throughput but in some circumstances, determinism is more 
desirable than aggregate throughput.

This patch essentially makes guest execution non-work conserving by 
making it appear to the scheduler that each guest wants 100% CPU even 
though they may be idling.

That means that regardless of what each guest is doing, if you have four 
guests on one CPU, each will get ~25% CPU[1].

[1] there are corner cases around things like forced sleep due to PFs 
and the like.  The goal is not for 100% determinism but more to at least 
obtain more significantly more determinism than we have now.

Regards,

Anthony Liguori

> thanks.
>
>
> 2010/12/2 Anthony Liguori<aliguori@us.ibm.com>:
>    
>> 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
>>
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>>      
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