From 229725acfaea0dd81f245aec40986dc766de40fa Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 28 Jul 2023 18:15:51 -0700 Subject: [PATCH] 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 --- 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 7217f75..b66af29 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/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. */ -- 2.7.4