The TPR_THRESHOLD field in the VMCS is used by VMX to induce VM exits when the guest's virtual TPR falls under the specified threshold, allowing KVM to inject previously masked interrupts. KVM handles these VM exits in handle_tpr_below_threshold(). Commit eb90f3417a0c ("KVM: vmx: speed up TPR below threshold vmexits") optimized this function by calling apic_update_ppr() instead of raising KVM_REQ_EVENT. apic_update_ppr() then raises KVM_REQ_EVENT if there is a pending, deliverable interrupt. However, if there are no new interrupts pending, apic_update_ppr() does not issue the request. This skips calling update_cr8_intercept(), and thus vmx_update_cr8_intercept() before VM entry, which results in a high, stale TPR_THRESHOLD. This is problematic due to the following sentence in 28.2.1.1 "VM-Execution Control Fields" in the SDM: The following check is performed if the “use TPR shadow” VM-execution control is 1 and the “virtualize APIC accesses” and “virtual-interrupt delivery” VM-execution controls are both 0: the value of bits 3:0 of the TPR threshold VM-execution control field should not be greater than the value of bits 7:4 of VTPR. This error condition is typically not observed when KVM runs on a bare metal system because modern processors support APICv, which enables virtual-interrupt delivery, and which KVM uses when possible. This causes the processor to no longer generate TPR-below threshold exits and to no longer check TPR_THRESHOLD on entry. However, when running on older platforms, or under nested virtualization on a hypervisor that does not support virtual-interrupt delivery and enforces this check (like Hyper-V) this can cause a VM entry failure with hardware error 0x7, as seen in [1]. Fix this by re-introducing an unconditional KVM_REQ_EVENT when reacting to a TPR-below-threshold exit, ensuring that vmx_update_cr8_intercept() is called to re-evaluate TPR_THRESHOLD before entering the guest. Link: https://github.com/coconut-svsm/svsm/issues/1081 [1] Tested-by: Stefano Garzarella Cc: stable@vger.kernel.org Fixes: eb90f3417a0c ("KVM: vmx: speed up TPR below threshold vmexits") Signed-off-by: Carlos López --- arch/x86/kvm/vmx/vmx.c | 1 + 1 file changed, 1 insertion(+) diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index c548f22375ad..21a469d3ba21 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -5824,6 +5824,7 @@ void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val) static int handle_tpr_below_threshold(struct kvm_vcpu *vcpu) { kvm_apic_update_ppr(vcpu); + kvm_make_request(KVM_REQ_EVENT, vcpu); return 1; } base-commit: c1f7303302927f9cbf4efedf70f0512cde168c65 -- 2.51.0