[PATCH -v2] vmx: increase ple_gap default to 64

Rik van Riel
2011-01-13T05:28:03Z

On some CPUs, a ple_gap of 41 is simply insufficient to ever trigger
PLE exits, even with the minimalistic PLE test from kvm-unit-tests.

http://git.kernel.org/?p=virt/kvm/kvm-unit-tests.git;a=commitdiff;h=eda71b28fa122203e316483b35f37aaacd42f545

For example, the Xeon X5670 CPU needs a ple_gap of at least 48 in
order to get pause loop exits:

# modprobe kvm_intel ple_gap=47
# taskset 1 /usr/local/bin/qemu-system-x86_64 -device testdev,chardev=log -chardev stdio,id=log -kernel x86/vmexit.flat -append ple-round-robin -smp 2
VNC server running on `::1:5900'
enabling apic
enabling apic
ple-round-robin 58298446
# rmmod kvm_intel
# modprobe kvm_intel ple_gap=48
# taskset 1 /usr/local/bin/qemu-system-x86_64 -device testdev,chardev=log -chardev stdio,id=log -kernel x86/vmexit.flat -append ple-round-robin -smp 2
VNC server running on `::1:5900'
enabling apic
enabling apic
ple-round-robin 36616

Increase the ple_gap to 128 to be on the safe side. Is this enough
for a CPU with HT that has a busy sibling thread, or should it be
even larger? On the X5670, loading up the sibling thread with an
infinite loop does not seem to increase the required ple_gap.

Signed-off-by: Rik van Riel <riel@redhat.com>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 81fcbe9..c61fcbf 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -90,14 +90,14 @@ module_param(vmm_exclusive, bool, S_IRUGO);
* These 2 parameters are used to config the controls for Pause-Loop Exiting:
* ple_gap: upper bound on the amount of time between two successive
* executions of PAUSE in a loop. Also indicate if ple enabled.
- * According to test, this time is usually small than 41 cycles.
+ * According to test, this time is usually smaller than 128 cycles.
* ple_window: upper bound on the amount of time a guest is allowed to execute
* in a PAUSE loop. Tests indicate that most spinlocks are held for
* less than 2^12 cycles
* Time is measured based on a counter that runs at the same rate as the TSC,
* refer SDM volume 3b section 21.6.13 & 22.1.3.
*/
-#define KVM_VMX_DEFAULT_PLE_GAP 41
+#define KVM_VMX_DEFAULT_PLE_GAP 128
#define KVM_VMX_DEFAULT_PLE_WINDOW 4096
static int ple_gap = KVM_VMX_DEFAULT_PLE_GAP;
module_param(ple_gap, int, S_IRUGO);