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.

Link: https://lore.kernel.org/r/20230729011608.1065019-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/svm.c

@@ -569,15 +569,11 @@ 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 inline void kvm_cpu_svm_disable(void)
@@ -1152,7 +1148,9 @@ 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. */