From e3417ab75ab2e7dca6372a1bfa26b1be3ac5889e Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Mon, 5 May 2025 11:03:00 -0700
Subject: [PATCH] KVM: SVM: Set/clear SRSO's BP_SPEC_REDUCE on 0 <=> 1 VM count
transitions
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.
Defer setting BP_SPEC_REDUCE until VMRUN is imminent to avoid impacting
performance on CPUs that aren't running VMs, e.g. if a setup is using
housekeeping CPUs. Setting BP_SPEC_REDUCE in task context, i.e. without
blasting IPIs to all CPUs, also helps avoid serializing 1<=>N transitions
without incurring a gross amount of complexity (see the Link for details
on how ugly coordinating via IPIs gets).
Link: https://lore.kernel.org/all/aBOnzNCngyS_pQIW@google.com
Fixes: 8442df2b49ed ("x86/bugs: KVM: Add support for SRSO_MSR_FIX")
Reported-by: Michael Larabel <Michael@michaellarabel.com>
Closes: https://www.phoronix.com/review/linux-615-amd-regression
Cc: Borislav Petkov <bp@alien8.de>
Tested-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250505180300.973137-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/svm.c | 71 ++++++++++++++++++++++++++++++++++++++----
arch/x86/kvm/svm/svm.h | 2 ++
2 files changed, 67 insertions(+), 6 deletions(-)
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index c5470d842aed..a89c271a1951 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;
}
@@ -1518,6 +1512,63 @@ static void svm_vcpu_free(struct kvm_vcpu *vcpu)
__free_pages(virt_to_page(svm->msrpm), get_order(MSRPM_SIZE));
}
+#ifdef CONFIG_CPU_MITIGATIONS
+static DEFINE_SPINLOCK(srso_lock);
+static atomic_t srso_nr_vms;
+
+static void svm_srso_clear_bp_spec_reduce(void *ign)
+{
+ struct svm_cpu_data *sd = this_cpu_ptr(&svm_data);
+
+ if (!sd->bp_spec_reduce_set)
+ return;
+
+ msr_clear_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT);
+ sd->bp_spec_reduce_set = false;
+}
+
+static void svm_srso_vm_destroy(void)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))
+ return;
+
+ if (atomic_dec_return(&srso_nr_vms))
+ return;
+
+ guard(spinlock)(&srso_lock);
+
+ /*
+ * Verify a new VM didn't come along, acquire the lock, and increment
+ * the count before this task acquired the lock.
+ */
+ if (atomic_read(&srso_nr_vms))
+ return;
+
+ on_each_cpu(svm_srso_clear_bp_spec_reduce, NULL, 1);
+}
+
+static void svm_srso_vm_init(void)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))
+ return;
+
+ /*
+ * Acquire the lock on 0 => 1 transitions to ensure a potential 1 => 0
+ * transition, i.e. destroying the last VM, is fully complete, e.g. so
+ * that a delayed IPI doesn't clear BP_SPEC_REDUCE after a vCPU runs.
+ */
+ if (atomic_inc_not_zero(&srso_nr_vms))
+ return;
+
+ guard(spinlock)(&srso_lock);
+
+ atomic_inc(&srso_nr_vms);
+}
+#else
+static void svm_srso_vm_init(void) { }
+static void svm_srso_vm_destroy(void) { }
+#endif
+
static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
{
struct vcpu_svm *svm = to_svm(vcpu);
@@ -1550,6 +1601,11 @@ static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
(!boot_cpu_has(X86_FEATURE_V_TSC_AUX) || !sev_es_guest(vcpu->kvm)))
kvm_set_user_return_msr(tsc_aux_uret_slot, svm->tsc_aux, -1ull);
+ if (cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE) &&
+ !sd->bp_spec_reduce_set) {
+ sd->bp_spec_reduce_set = true;
+ msr_set_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_BP_SPEC_REDUCE_BIT);
+ }
svm->guest_state_loaded = true;
}
@@ -5040,6 +5096,8 @@ static void svm_vm_destroy(struct kvm *kvm)
{
avic_vm_destroy(kvm);
sev_vm_destroy(kvm);
+
+ svm_srso_vm_destroy();
}
static int svm_vm_init(struct kvm *kvm)
@@ -5065,6 +5123,7 @@ static int svm_vm_init(struct kvm *kvm)
return ret;
}
+ svm_srso_vm_init();
return 0;
}
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index d4490eaed55d..f16b068c4228 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -335,6 +335,8 @@ struct svm_cpu_data {
u32 next_asid;
u32 min_asid;
+ bool bp_spec_reduce_set;
+
struct vmcb *save_area;
unsigned long save_area_pa;