x86/KVM/VMX: Use MSR save list for IA32_FLUSH_CMD if required

If the L1D flush module parameter is set to 'always' and the IA32_FLUSH_CMD
MSR is available, optimize the VMENTER code with the MSR save list.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kvm/vmx.c

@@ -6237,6 +6237,16 @@ static void ept_set_mmio_spte_mask(void)
 				   VMX_EPT_MISCONFIG_WX_VALUE);
 }
 
+static bool vmx_l1d_use_msr_save_list(void)
+{
+	if (!enable_ept || !boot_cpu_has_bug(X86_BUG_L1TF) ||
+	    static_cpu_has(X86_FEATURE_HYPERVISOR) ||
+	    !static_cpu_has(X86_FEATURE_FLUSH_L1D))
+		return false;
+
+	return vmentry_l1d_flush == VMENTER_L1D_FLUSH_ALWAYS;
+}
+
 #define VMX_XSS_EXIT_BITMAP 0
 /*
  * Sets up the vmcs for emulated real mode.
@@ -6358,6 +6368,12 @@ static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
 		vmcs_write64(PML_ADDRESS, page_to_phys(vmx->pml_pg));
 		vmcs_write16(GUEST_PML_INDEX, PML_ENTITY_NUM - 1);
 	}
+	/*
+	 * If flushing the L1D cache on every VMENTER is enforced and the
+	 * MSR is available, use the MSR save list.
+	 */
+	if (vmx_l1d_use_msr_save_list())
+		add_atomic_switch_msr(vmx, MSR_IA32_FLUSH_CMD, L1D_FLUSH, 0, true);
 }
 
 static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
@@ -9607,11 +9623,26 @@ static void vmx_l1d_flush(struct kvm_vcpu *vcpu)
 	bool always;
 
 	/*
-	 * If the mitigation mode is 'flush always', keep the flush bit
-	 * set, otherwise clear it. It gets set again either from
-	 * vcpu_run() or from one of the unsafe VMEXIT handlers.
+	 * This code is only executed when:
+	 * - the flush mode is 'cond'
+	 * - the flush mode is 'always' and the flush MSR is not
+	 *   available
+	 *
+	 * If the CPU has the flush MSR then clear the flush bit because
+	 * 'always' mode is handled via the MSR save list.
+	 *
+	 * If the MSR is not avaibable then act depending on the mitigation
+	 * mode: If 'flush always', keep the flush bit set, otherwise clear
+	 * it.
+	 *
+	 * The flush bit gets set again either from vcpu_run() or from one
+	 * of the unsafe VMEXIT handlers.
 	 */
+	if (static_cpu_has(X86_FEATURE_FLUSH_L1D))
+		always = false;
+	else
 		always = vmentry_l1d_flush == VMENTER_L1D_FLUSH_ALWAYS;
+
 	vcpu->arch.l1tf_flush_l1d = always;
 
 	vcpu->stat.l1d_flush++;
@@ -13205,7 +13236,8 @@ static int __init vmx_setup_l1d_flush(void)
 	struct page *page;
 
 	if (vmentry_l1d_flush == VMENTER_L1D_FLUSH_NEVER ||
-	    !boot_cpu_has_bug(X86_BUG_L1TF))
+	    !boot_cpu_has_bug(X86_BUG_L1TF) ||
+	    vmx_l1d_use_msr_save_list())
 		return 0;
 
 	if (!boot_cpu_has(X86_FEATURE_FLUSH_L1D)) {