[PATCH] KVM: SVM: Set/clear SRSO's BP_SPEC_REDUCE on 0 <=> 1 VM count transitions

[Sean Christopherson <seanjc@google.com>]

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@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>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 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