Hi Jinank,

On Thu, 03 Jun 2021 12:05:54 +0100,
Jinank Jain [off-list ref] wrote:

Currently if a guest is live-migrated while it is actively using perf
counters, then after live-migrate it will notice that all counters would
suddenly start reporting 0s. This is due to the fact we are not
re-creating the relevant perf events inside the kernel.

Usually on live-migration guest state is restored using KVM_SET_ONE_REG
ioctl interface, which simply restores the value of PMU registers
values but does not re-program the perf events so that the guest can seamlessly
use these counters even after live-migration like it was doing before
live-migration.

Instead there are two completely different code path between guest
accessing PMU registers and VMM restoring counters on
live-migration.

In case of KVM_SET_ONE_REG:

kvm_arm_set_reg()
...... kvm_arm_sys_reg_set_reg()
........... reg_from_user()

but in case when guest tries to access these counters:

handle_exit()
..... kvm_handle_sys_reg()
..........perform_access()
...............access_pmu_evcntr()
...................kvm_pmu_set_counter_value()
.......................kvm_pmu_create_perf_event()

The drawback of using the KVM_SET_ONE_REG interface is that the host pmu
events which were registered for the source instance and not present for
the destination instance.

I can't parse this sentence. Do you mean "are not present"?

If this only needs to be run once per vcpu, why not trigger it from
kvm_arm_pmu_v3_enable(), which is also called once per vcpu?

This can done on the back of a request, saving most of the overhead
and not requiring any extra field. Essentially, something like the
(untested) patch below.

Why? There is no architectural guarantee that a counter resets to 0
without writing PMCR_EL0.C. And if you want the guest to continue
counting where it left off, resetting the counter is at best
counter-productive.

So I must be missing something...

 		kvm_pmu_set_counter_value(vcpu, ARMV8_PMU_CYCLE_IDX, 0);
 
-	if (val & ARMV8_PMU_PMCR_P) {
+	/*
+	 * All the counters needs to reset in case of first vcpu load.
+	 */
+	if (val & ARMV8_PMU_PMCR_P || !kvm_arm_pmu_v3_restored(vcpu)) {

Same thing here.

 		for_each_set_bit(i, &mask, 32)
 			kvm_pmu_set_counter_value(vcpu, i, 0);
 	}

The rest of the changes should be unnecessary with the patch below.

Thanks,

	M.

From 1be188ab71867632a8c17384be6e55f47f42aa8b Mon Sep 17 00:00:00 2001
From: Marc Zyngier <maz@kernel.org>
Date: Thu, 3 Jun 2021 16:50:02 +0100
Subject: [PATCH] KVM: arm64: Restore PMU configuration on first run

Restoring a guest with an active virtual PMU results in no perf
counters being instanciated on the host side. Not quite what
you'd expect from a restore.

In order to fix this, force a writeback of PMCR_EL0 on the first
run of a vcpu (using a new request so that it happens once the
vcpu has been loaded). This will in turn create all the host-side
counters that were missing.

Reported-by: Jinank Jain <redacted>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/include/asm/kvm_host.h | 1 +
 arch/arm64/kvm/arm.c              | 4 ++++
 arch/arm64/kvm/pmu-emul.c         | 3 +++
 3 files changed, 8 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 7cd7d5c8c4bc..6336b4309114 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h

@@ -46,6 +46,7 @@
 #define KVM_REQ_VCPU_RESET	KVM_ARCH_REQ(2)
 #define KVM_REQ_RECORD_STEAL	KVM_ARCH_REQ(3)
 #define KVM_REQ_RELOAD_GICv4	KVM_ARCH_REQ(4)
+#define KVM_REQ_RELOAD_PMU	KVM_ARCH_REQ(5)
 
 #define KVM_DIRTY_LOG_MANUAL_CAPS   (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | \
 				     KVM_DIRTY_LOG_INITIALLY_SET)

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index e720148232a0..facf4d41d32a 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c

@@ -689,6 +689,10 @@ static void check_vcpu_requests(struct kvm_vcpu *vcpu)
 			vgic_v4_load(vcpu);
 			preempt_enable();
 		}
+
+		if (kvm_check_request(KVM_REQ_RELOAD_PMU, vcpu))
+			kvm_pmu_handle_pmcr(vcpu,
+					    __vcpu_sys_reg(vcpu, PMCR_EL0));
 	}
 }
 

diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index fd167d4f4215..a0bbb7111f57 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c

@@ -850,6 +850,9 @@ int kvm_arm_pmu_v3_enable(struct kvm_vcpu *vcpu)
 		   return -EINVAL;
 	}
 
+	/* One-off reload of the PMU on first run */
+	kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
+
 	return 0;
 }
 

-- 
2.30.2


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

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