linux-kernel - [PATCH 12/14] KVM: x86: SVM: add module param to control TSC scaling

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Date:   Tue, 14 Sep 2021 18:48:23 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>, Bandan Das <bsd@...hat.com>,
        linux-kernel@...r.kernel.org (open list),
        Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
        Wei Huang <wei.huang2@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kselftest@...r.kernel.org (open list:KERNEL SELFTEST FRAMEWORK),
        Maxim Levitsky <mlevitsk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        x86@...nel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)),
        Shuah Khan <shuah@...nel.org>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: [PATCH 12/14] KVM: x86: SVM: add module param to control TSC scaling

This allows to easily simulate a CPU without this feature.

Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
---
 arch/x86/kvm/svm/svm.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 6504e40e0985..001a5af842ba 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -191,6 +191,9 @@ static int lbrv = true;
 module_param(lbrv, int, 0444);
 
 
+static int tsc_scaling = true;
+module_param(tsc_scaling, int, 0444);
+
 /*
  * enable / disable AVIC.  Because the defaults differ for APICv
  * support between VMX and SVM we cannot use module_param_named.
@@ -471,7 +474,7 @@ static int has_svm(void)
 static void svm_hardware_disable(void)
 {
 	/* Make sure we clean up behind us */
-	if (static_cpu_has(X86_FEATURE_TSCRATEMSR))
+	if (tsc_scaling)
 		wrmsrl(MSR_AMD64_TSC_RATIO, TSC_RATIO_DEFAULT);
 
 	cpu_svm_disable();
@@ -514,6 +517,10 @@ static int svm_hardware_enable(void)
 	wrmsrl(MSR_VM_HSAVE_PA, __sme_page_pa(sd->save_area));
 
 	if (static_cpu_has(X86_FEATURE_TSCRATEMSR)) {
+		/*
+		 * Set the default value, even if we don't use TSC scaling
+		 * to avoid having stale value in the msr
+		 */
 		wrmsrl(MSR_AMD64_TSC_RATIO, TSC_RATIO_DEFAULT);
 		__this_cpu_write(current_tsc_ratio, TSC_RATIO_DEFAULT);
 	}
@@ -1063,10 +1070,15 @@ static __init int svm_hardware_setup(void)
 	if (boot_cpu_has(X86_FEATURE_FXSR_OPT))
 		kvm_enable_efer_bits(EFER_FFXSR);
 
-	if (boot_cpu_has(X86_FEATURE_TSCRATEMSR)) {
-		kvm_has_tsc_control = true;
-		kvm_max_tsc_scaling_ratio = TSC_RATIO_MAX;
-		kvm_tsc_scaling_ratio_frac_bits = 32;
+	if (tsc_scaling) {
+		if (!boot_cpu_has(X86_FEATURE_TSCRATEMSR)) {
+			tsc_scaling = false;
+		} else {
+			pr_info("TSC scaling supported\n");
+			kvm_has_tsc_control = true;
+			kvm_max_tsc_scaling_ratio = TSC_RATIO_MAX;
+			kvm_tsc_scaling_ratio_frac_bits = 32;
+		}
 	}
 
 	tsc_aux_uret_slot = kvm_add_user_return_msr(MSR_TSC_AUX);
@@ -1543,7 +1555,7 @@ static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu)
 		vmsave(__sme_page_pa(sd->save_area));
 	}
 
-	if (static_cpu_has(X86_FEATURE_TSCRATEMSR)) {
+	if (tsc_scaling) {
 		u64 tsc_ratio = vcpu->arch.tsc_scaling_ratio;
 		if (tsc_ratio != __this_cpu_read(current_tsc_ratio)) {
 			__this_cpu_write(current_tsc_ratio, tsc_ratio);
-- 
2.26.3