[PATCH] kvm-vmx: add module parameter to avoid trapping HLT instructions

[PATCH] kvm-vmx: add module parameter to avoid trapping HLT instructions

[Anthony Liguori]

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>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index caa967e..f59bdb3 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 __read_mostly enable_yield_on_guest_hlt = 1;
+module_param(enable_yield_on_guest_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 (enable_yield_on_guest_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

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

[Avi Kivity]

On 12/01/2010 08:03 PM, 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.

Looks reasonable.

> +static int __read_mostly enable_yield_on_guest_hlt = 1;
> +module_param(enable_yield_on_guest_hlt, bool, S_IRUGO);
> +

Please rename to yield_on_hlt.  Also, should not be __read_mostly 
(__read_mostly is for variables that are also __read_often).


-- 
error compiling committee.c: too many arguments to function