VMX-Preemption Timer

VMX-Preemption Timer の動作

有効化されると、 VMX non-root operation 中にカウントダウンする

カウンタの初期値は VM Entry 時にロードされた値

タイマーが0になると VM Exit を起こす

カウントダウンは TSC に比例したレートで実施される

TSC がインクリメントされる際、値 X（0~31） に応じてタイマーが 1 減少

cpu state

C0、C1、C2で動作

shutdown, wait-fot-SIPI state でも動作

C2 より深い CPU state ではカウントダウンしない

0になった時の動作

論理プロセッサは C0 C-state に遷移しVM exit を引き起こす

ただしwait-for-SIPI 状態の場合のみ 0 になっても VM exit は発生しない

SDM Vol-3C

27.5.1 VMX-Preemption Timer

If the last VM entry was performed with the 1-setting of “activate VMX-preemption timer” VM-execution control, the VMX-preemption timer counts down (from the value loaded by VM entry; see Section 28.7.4) in VMX non-root operation. When the timer counts down to zero, it stops counting down and a VM exit occurs (see Section 27.2).

The VMX-preemption timer counts down at rate proportional to that of the timestamp counter (TSC). Specifically, the timer counts down by 1 every time bit X in the TSC changes due to a TSC increment. The value of X is in the range 0–31 and can be determined by consulting the VMX capability MSR IA32_VMX_MISC (see Appendix A.6).

The VMX-preemption timer operates in the C-states C0, C1, and C2; it also operates in the shutdown and wait-for-SIPI states. If the timer counts down to zero in any state other than the wait-for SIPI state, the logical processor transitions to the C0 C-state and causes a VM exit; the timer does not cause a VM exit if it counts down to zero in the wait-for-SIPI state. The timer is not decremented in C-states deeper than C2.

Treatment of the timer in the case of system management interrupts (SMIs) and system-management mode (SMM) depends on whether the treatment of SMIs and SMM:

If the default treatment of SMIs and SMM (see Section 33.14) is active, the VMX-preemption timer counts across an SMI to VMX non-root operation, subsequent execution in SMM, and the return from SMM via the RSM instruction. However, the timer can cause a VM exit only from VMX non-root operation. If the timer expires during SMI, in SMM, or during RSM, a timer-induced VM exit occurs immediately after RSM with its normal priority unless it is blocked based on activity state (Section 27.2).

If the dual-monitor treatment of SMIs and SMM (see Section 33.15) is active, transitions into and out of SMM are VM exits and VM entries, respectively. The treatment of the VMX-preemption timer by those transitions is mostly the same as for ordinary VM exits and VM entries; Section 33.15.2 and Section 33.15.4 detail some differences.