linux-kernel - [PATCH v2 04/21] KVM: SVM: Clean up preemption toggling related to MSR_AMD64_TSC

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Message-ID: <20230729011608.1065019-5-seanjc@google.com>
Date:   Fri, 28 Jul 2023 18:15:51 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH v2 04/21] KVM: SVM: Clean up preemption toggling related to MSR_AMD64_TSC_RATIO

Explicitly disable preemption when writing MSR_AMD64_TSC_RATIO only in the
"outer" helper, as all direct callers of the "inner" helper now run with
preemption already disabled.  And that isn't a coincidence, as the outer
helper requires a vCPU and is intended to be used when modifying guest
state and/or emulating guest instructions, which are typically done with
preemption enabled.

Direct use of the inner helper should be extremely limited, as the only
time KVM should modify MSR_AMD64_TSC_RATIO without a vCPU is when
sanitizing the MSR for a specific pCPU (currently done when {en,dis}abling
disabling SVM). The other direct caller is svm_prepare_switch_to_guest(),
which does have a vCPU, but is a one-off special case: KVM is about to
enter the guest on a specific pCPU and thus must have preemption disabled.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/svm.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 13f316375b14..9fc5e402636a 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -552,15 +552,11 @@ static int svm_check_processor_compat(void)

 static void __svm_write_tsc_multiplier(u64 multiplier)
 {
-	preempt_disable();
-
 	if (multiplier == __this_cpu_read(current_tsc_ratio))
-		goto out;
+		return;

 	wrmsrl(MSR_AMD64_TSC_RATIO, multiplier);
 	__this_cpu_write(current_tsc_ratio, multiplier);
-out:
-	preempt_enable();
 }

 static void svm_hardware_disable(void)
@@ -1112,7 +1108,9 @@ static void svm_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)

 void svm_write_tsc_multiplier(struct kvm_vcpu *vcpu, u64 multiplier)
 {
+	preempt_disable();
 	__svm_write_tsc_multiplier(multiplier);
+	preempt_enable();
 }

 /* Evaluate instruction intercepts that depend on guest CPUID features. */
-- 
2.41.0.487.g6d72f3e995-goog