On Mon, 20 Sep 2021 10:56:57 +0100,
Will Deacon [off-list ref] wrote:

On Sun, Sep 19, 2021 at 02:36:46PM +0100, Marc Zyngier wrote:

quoted

From 9c26e3e6bbcbc3a583b3974e7a9017029d31fe29 Mon Sep 17 00:00:00 2001
From: Marc Zyngier <maz@kernel.org>
Date: Sun, 19 Sep 2021 14:09:49 +0100
Subject: [PATCH] KVM: arm64: Fix PMU probe ordering

Russell reported that since 5.13, KVM's probing of the PMU has
started to fail on his HW. As it turns out, there is an implicit
ordering dependency between the architectural PMU probing code and
and KVM's own probing. If, due to probe ordering reasons, KVM probes
before the PMU driver, it will fail to detect the PMU and prevent it
from being advertised to guests as well as the VMM.

Obviously, this is one probing too many, and we should be able to
deal with any ordering.

Add a callback from the PMU code into KVM to advertise the registration
of a host CPU PMU, allowing for any probing order.

Fixes: 5421db1be3b1 ("KVM: arm64: Divorce the perf code from oprofile helpers")
Reported-by: "Russell King (Oracle)" <linux@armlinux.org.uk>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/YUYRKVflRtUytzy5@shell.armlinux.org.uk (local)
Cc: stable@vger.kernel.org
---
 arch/arm64/kvm/perf.c        |  3 ---
 arch/arm64/kvm/pmu-emul.c    | 12 +++++++++++-
 drivers/perf/arm_pmu.c       |  2 ++
 include/kvm/arm_pmu.h        |  3 ---
 include/linux/perf/arm_pmu.h |  6 ++++++
 5 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kvm/perf.c b/arch/arm64/kvm/perf.c
index f9bb3b14130e..c84fe24b2ea1 100644
--- a/arch/arm64/kvm/perf.c
+++ b/arch/arm64/kvm/perf.c

@@ -50,9 +50,6 @@ static struct perf_guest_info_callbacks kvm_guest_cbs = {
 
 int kvm_perf_init(void)
 {
-	if (kvm_pmu_probe_pmuver() != ID_AA64DFR0_PMUVER_IMP_DEF && !is_protected_kvm_enabled())
-		static_branch_enable(&kvm_arm_pmu_available);
-
 	return perf_register_guest_info_callbacks(&kvm_guest_cbs);
 }
 

diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index f5065f23b413..588100c52f34 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c

@@ -740,7 +740,17 @@ void kvm_pmu_set_counter_event_type(struct kvm_vcpu *vcpu, u64 data,
 	kvm_pmu_create_perf_event(vcpu, select_idx);
 }
 
-int kvm_pmu_probe_pmuver(void)
+void kvm_host_pmu_init(struct arm_pmu *pmu)
+{
+	if (pmu->pmuver != 0 &&
+	    pmu->pmuver != ID_AA64DFR0_PMUVER_IMP_DEF &&
+	    !is_protected_kvm_enabled()) {
+		static_branch_enable(&kvm_arm_pmu_available);
+		kvm_info("PMU detected and enabled\n");
+	}
+}
+
+static int kvm_pmu_probe_pmuver(void)
 {
 	struct perf_event_attr attr = { };
 	struct perf_event *event;

diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
index 3cbc3baf087f..295cc7952d0e 100644
--- a/drivers/perf/arm_pmu.c
+++ b/drivers/perf/arm_pmu.c

@@ -952,6 +952,8 @@ int armpmu_register(struct arm_pmu *pmu)
 		pmu->name, pmu->num_events,
 		has_nmi ? ", using NMIs" : "");
 
+	kvm_host_pmu_init(pmu);

Just a nit, but I think this will get called for each PMU we probe
on a big.LITTLE system which is probably harmless, but possible not
what you want?

Yeah, it is a bit ugly, but harmless. In the future, it would be
useful to track which PMU is used on which CPUs, and this will give us
a decent hook.

I'll tone the print down though.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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