The VMX-preemption timer is used by KVM as a way to set deadlines for the guest (i.e. timer emulation). That was safe till very recently when capability KVM_X86_DISABLE_EXITS_MWAIT to disable intercepting MWAIT was introduced. According to Intel SDM 25.5.1: """ 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. """ Now once the guest issues the MWAIT with a c-state deeper than C2 the preemption timer will never wake it up again since it stopped ticking! Usually this is compensated by other activities in the system that would wake the core from the deep C-state (and cause a VMExit). For example, if the host itself is ticking or it received interrupts, etc! So disable the VMX-preemption timer is MWAIT is exposed to the guest! Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: x86@xxxxxxxxxx Cc: kvm@xxxxxxxxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx> --- v1 -> v2: - Drop everything .. just return -EOPNOTSUPP (pbonzini@) :D --- arch/x86/kvm/vmx.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index d2e54e7..d99008b 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -11908,6 +11908,9 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc) u64 guest_tscl = kvm_read_l1_tsc(vcpu, tscl); u64 delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl; + if (kvm_pause_in_guest(vcpu->kvm)) + return -EOPNOTSUPP; + /* Convert to host delta tsc if tsc scaling is enabled */ if (vcpu->arch.tsc_scaling_ratio != kvm_default_tsc_scaling_ratio && u64_shl_div_u64(delta_tsc, -- 2.7.4
