[PATCH v7 22/32] KVM: PPC: Book3S HV: Remove support for dependent threads mode on P9

Nicholas Piggin npiggin at gmail.com
Fri May 28 19:07:42 AEST 2021

Dependent-threads mode is the normal KVM mode for pre-POWER9 SMT
processors, where all threads in a core (or subcore) would run the same
partition at the same time, or they would run the host.

This design was mandated by MMU state that is shared between threads in
a processor, so the synchronisation point is in hypervisor real-mode
that has essentially no shared state, so it's safe for multiple threads
to gather and switch to the correct mode.

It is implemented by having the host unplug all secondary threads and
always run in SMT1 mode, and host QEMU threads essentially represent
virtual cores that wake these secondary threads out of unplug when the
ioctl is called to run the guest. This happens via a side-path that is
mostly invisible to the rest of the Linux host and the secondary threads
still appear to be unplugged.

POWER9 / ISA v3.0 has a more flexible MMU design that is independent
per-thread and allows a much simpler KVM implementation. Before the new
"P9 fast path" was added that began to take advantage of this, POWER9
support was implemented in the existing path which has support to run
in the dependent threads mode. So it was not much work to add support to
run POWER9 in this dependent threads mode.

The mode is not required by the POWER9 MMU (although "mixed-mode" hash /
radix MMU limitations of early processors were worked around using this
mode). But it is one way to run SMT guests without running different
guests or guest and host on different threads of the same core, so it
could avoid or reduce some SMT attack surfaces without turning off SMT

This security feature has some real, if indeterminate, value. However
the old path is lagging in features (nested HV), and with this series
the new P9 path adds remaining missing features (radix prefetch bug
and hash support, in later patches), so POWER9 dependent threads mode
support would be the only remaining reason to keep that code in and keep
supporting POWER9/POWER10 in the old path. So here we make the call to
drop this feature.

Remove dependent threads mode support for POWER9 and above processors.
Systems can still achieve this security by disabling SMT entirely, but
that would generally come at a larger performance cost for guests.

Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
 arch/powerpc/include/asm/kvm_asm.h  |  2 +-
 arch/powerpc/include/asm/kvm_host.h |  1 -
 arch/powerpc/kvm/book3s_64_entry.S  |  3 +--
 arch/powerpc/kvm/book3s_hv.c        | 27 +++++----------------------
 4 files changed, 7 insertions(+), 26 deletions(-)

diff --git a/arch/powerpc/include/asm/kvm_asm.h b/arch/powerpc/include/asm/kvm_asm.h
index 43b1788e1f93..f4ae37810aa9 100644
--- a/arch/powerpc/include/asm/kvm_asm.h
+++ b/arch/powerpc/include/asm/kvm_asm.h
@@ -147,7 +147,7 @@
-#define KVM_GUEST_MODE_HV_FAST	5 /* ISA >= v3.0 host+guest radix, indep thr */
+#define KVM_GUEST_MODE_HV_FAST	5 /* ISA >= v3.0 host+guest radix */
diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
index 69add9d662df..6904ce9e8190 100644
--- a/arch/powerpc/include/asm/kvm_host.h
+++ b/arch/powerpc/include/asm/kvm_host.h
@@ -297,7 +297,6 @@ struct kvm_arch {
 	u8 fwnmi_enabled;
 	u8 secure_guest;
 	u8 svm_enabled;
-	bool threads_indep;
 	bool nested_enable;
 	bool dawr1_enabled;
 	pgd_t *pgtable;
diff --git a/arch/powerpc/kvm/book3s_64_entry.S b/arch/powerpc/kvm/book3s_64_entry.S
index 177e8fad5c8d..bac664c1a9f7 100644
--- a/arch/powerpc/kvm/book3s_64_entry.S
+++ b/arch/powerpc/kvm/book3s_64_entry.S
  * void kvmppc_p9_enter_guest(struct vcpu *vcpu);
  * Enter the guest on a ISAv3.0 or later system where we have exactly
- * one vcpu per vcore, and both the host and guest are radix, and threads
- * are set to "indepdent mode".
+ * one vcpu per vcore, and both the host and guest are radix.
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index eb25605e23b9..acb0c72ea900 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -103,13 +103,9 @@ static int target_smt_mode;
 module_param(target_smt_mode, int, 0644);
 MODULE_PARM_DESC(target_smt_mode, "Target threads per core (0 = max)");
-static bool indep_threads_mode = true;
-module_param(indep_threads_mode, bool, S_IRUGO | S_IWUSR);
-MODULE_PARM_DESC(indep_threads_mode, "Independent-threads mode (only on POWER9)");
 static bool one_vm_per_core;
 module_param(one_vm_per_core, bool, S_IRUGO | S_IWUSR);
-MODULE_PARM_DESC(one_vm_per_core, "Only run vCPUs from the same VM on a core (requires indep_threads_mode=N)");
+MODULE_PARM_DESC(one_vm_per_core, "Only run vCPUs from the same VM on a core (requires POWER8 or older)");
 static const struct kernel_param_ops module_param_ops = {
@@ -2265,7 +2261,7 @@ static int kvmppc_set_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,
 static int threads_per_vcore(struct kvm *kvm)
-	if (kvm->arch.threads_indep)
+	if (cpu_has_feature(CPU_FTR_ARCH_300))
 		return 1;
 	return threads_per_subcore;
@@ -4369,7 +4365,7 @@ static int kvmppc_vcpu_run_hv(struct kvm_vcpu *vcpu)
 	vcpu->arch.state = KVMPPC_VCPU_BUSY_IN_HOST;
 	do {
-		if (kvm->arch.threads_indep && kvm_is_radix(kvm))
+		if (kvm_is_radix(kvm))
 			r = kvmhv_run_single_vcpu(vcpu, ~(u64)0,
@@ -4992,21 +4988,8 @@ static int kvmppc_core_init_vm_hv(struct kvm *kvm)
 	 * Track that we now have a HV mode VM active. This blocks secondary
 	 * CPU threads from coming online.
-	 * On POWER9, we only need to do this if the "indep_threads_mode"
-	 * module parameter has been set to N.
-	if (cpu_has_feature(CPU_FTR_ARCH_300)) {
-		if (!indep_threads_mode && !cpu_has_feature(CPU_FTR_HVMODE)) {
-			pr_warn("KVM: Ignoring indep_threads_mode=N in nested hypervisor\n");
-			kvm->arch.threads_indep = true;
-		} else if (!indep_threads_mode && cpu_has_feature(CPU_FTR_P9_RADIX_PREFETCH_BUG)) {
-			pr_warn("KVM: Ignoring indep_threads_mode=N on pre-DD2.2 POWER9\n");
-			kvm->arch.threads_indep = true;
-		} else {
-			kvm->arch.threads_indep = indep_threads_mode;
-		}
-	}
-	if (!kvm->arch.threads_indep)
+	if (!cpu_has_feature(CPU_FTR_ARCH_300))
@@ -5047,7 +5030,7 @@ static void kvmppc_core_destroy_vm_hv(struct kvm *kvm)
-	if (!kvm->arch.threads_indep)
+	if (!cpu_has_feature(CPU_FTR_ARCH_300))

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