Set the magic BP_SPEC_REDUCE bit to mitigate SRSO when running VMs if and only if KVM has at least one active VM. Leaving the bit set at all times unfortunately degrades performance by a wee bit more than expected. Use a dedicated spinlock and counter instead of hooking virtualization enablement, as changing the behavior of kvm.enable_virt_at_load based on SRSO_BP_SPEC_REDUCE is painful, and has its own drawbacks, e.g. could result in performance issues for flows that are sensitive to VM creation latency. Similarly, don't bother optimizing the 1=>N and N=>1 transitions, e.g. by using atomic_inc_return() to avoid taking the spinlock, as ensuring that BP_SPEC_REDUCE is guaranteed to be set before KVM_RUN is non-trivial. KVM already serializes VM creation against kvm_lock (to add the VM to vm_list), and the spinlock will only be held for a handful of cycles for the 1<=>N cases. I.e. the complexity needed to ensure correctness outweighs the marginal benefits of eliding the lock. See the Link for details. Link: https://lore.kernel.org/all/aBOnzNCngyS_pQIW@xxxxxxxxxx Fixes: 8442df2b49ed ("x86/bugs: KVM: Add support for SRSO_MSR_FIX") Reported-by: Michael Larabel <Michael@xxxxxxxxxxxxxxxxxx> Closes: https://www.phoronix.com/review/linux-615-amd-regression Cc: Borislav Petkov <bp@xxxxxxxxx> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> --- arch/x86/kvm/svm/svm.c | 43 ++++++++++++++++++++++++++++++++++++------ 1 file changed, 37 insertions(+), 6 deletions(-) diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index cc1c721ba067..364959fd1040 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -607,9 +607,6 @@ static void svm_disable_virtualization_cpu(void) kvm_cpu_svm_disable(); amd_pmu_disable_virt(); - - if (cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE)) - msr_clear_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT); } static int svm_enable_virtualization_cpu(void) @@ -687,9 +684,6 @@ static int svm_enable_virtualization_cpu(void) rdmsr(MSR_TSC_AUX, sev_es_host_save_area(sd)->tsc_aux, msr_hi); } - if (cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE)) - msr_set_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT); - return 0; } @@ -5032,10 +5026,46 @@ static void svm_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector) sev_vcpu_deliver_sipi_vector(vcpu, vector); } +#ifdef CONFIG_CPU_MITIGATIONS +static DEFINE_SPINLOCK(srso_lock); +static int srso_nr_vms; + +static void svm_toggle_srso_spec_reduce(void *set) +{ + if (set) + msr_set_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT); + else + msr_clear_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT); +} + +static void svm_srso_add_remove_vm(int count) +{ + bool set; + + if (!cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE)) + return; + + guard(spinlock)(&srso_lock); + + set = !srso_nr_vms; + srso_nr_vms += count; + + WARN_ON_ONCE(srso_nr_vms < 0); + if (!set && srso_nr_vms) + return; + + on_each_cpu(svm_toggle_srso_spec_reduce, (void *)set, 1); +} +#else +static void svm_srso_add_remove_vm(int count) { } +#endif + static void svm_vm_destroy(struct kvm *kvm) { avic_vm_destroy(kvm); sev_vm_destroy(kvm); + + svm_srso_add_remove_vm(-1); } static int svm_vm_init(struct kvm *kvm) @@ -5061,6 +5091,7 @@ static int svm_vm_init(struct kvm *kvm) return ret; } + svm_srso_add_remove_vm(1); return 0; } base-commit: 45eb29140e68ffe8e93a5471006858a018480a45 -- 2.49.0.906.g1f30a19c02-goog
