linux-kernel - [PATCH 22/32] KVM: VMX: Introduce module parameter for APIC virt timer support

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Message-ID: <aa16814721131adc3b133945a3437436fd544eb8.1770116051.git.isaku.yamahata@intel.com>
Date: Tue,  3 Feb 2026 10:17:05 -0800
From: isaku.yamahata@...el.com
To: kvm@...r.kernel.org
Cc: isaku.yamahata@...el.com,
	isaku.yamahata@...il.com,
	Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	linux-kernel@...r.kernel.org,
	Yang Zhong <yang.zhong@...ux.intel.com>
Subject: [PATCH 22/32] KVM: VMX: Introduce module parameter for APIC virt timer support

From: Yang Zhong <yang.zhong@...ux.intel.com>

Introduce a new module parameter, apic_timer_virt, to control the
virtualization of the APIC timer in KVM. The primary improvement offered by
APIC timer virtualization over the preemption timer is the passthrough of
MSR_IA32_TSC_DEADLINE to the VM. This passthrough capability reduces the
number of VM exits triggered by MSR write operations, thereby enhancing the
performance of the virtual machine.

The apic_timer_virt parameter is set to true by default on processor
platforms that support APIC timer virtualization.  On platforms that do not
support this feature, the parameter will indicate that APIC timer
virtualization is not available.

Signed-off-by: Yang Zhong <yang.zhong@...ux.intel.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
---
 arch/x86/kvm/vmx/vmx.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 5496f4230424..76725f8dd228 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -118,6 +118,9 @@ module_param(enable_ipiv, bool, 0444);
 
 module_param(enable_device_posted_irqs, bool, 0444);
 
+static bool __read_mostly enable_apic_timer_virt = true;
+module_param_named(apic_timer_virt, enable_apic_timer_virt, bool, 0444);
+
 /*
  * If nested=1, nested virtualization is supported, i.e., guests may use
  * VMX and be a hypervisor for its own guests. If nested=0, guests may not
@@ -2799,7 +2802,7 @@ static int setup_vmcs_config(struct vmcs_config *vmcs_conf,
 			adjust_vmx_controls64(KVM_OPTIONAL_VMX_TERTIARY_VM_EXEC_CONTROL,
 					      MSR_IA32_VMX_PROCBASED_CTLS3);
 
-	if (!IS_ENABLED(CONFIG_X86_64) ||
+	if (!IS_ENABLED(CONFIG_X86_64) || !enable_apic_timer_virt ||
 	    !(_cpu_based_2nd_exec_control & SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY))
 		_cpu_based_3rd_exec_control &= ~TERTIARY_EXEC_GUEST_APIC_TIMER;
 
@@ -8748,6 +8751,9 @@ __init int vmx_hardware_setup(void)
 	if (!cpu_has_vmx_preemption_timer())
 		enable_preemption_timer = false;
 
+	if (!cpu_has_vmx_apic_timer_virt())
+		enable_apic_timer_virt = false;
+
 	if (enable_preemption_timer) {
 		u64 use_timer_freq = 5000ULL * 1000 * 1000;
 
-- 
2.45.2