Hi Marc.

On Mon, 2021-06-07 at 17:35 +0100, Marc Zyngier wrote:

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On Mon, 07 Jun 2021 17:05:01 +0100,
"Jain, Jinank" [off-list ref] wrote:

quoted

On Thu, 2021-06-03 at 17:03 +0100, Marc Zyngier wrote:

quoted

Hi Jinank,

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

quoted

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"?

quoted

Thus passively restoring PMCR_EL0 using
KVM_SET_ONE_REG interface would not create the necessary host
pmu
events
which are crucial for seamless guest experience across live
migration.

In ordet to fix the situation, on first vcpu load we should
restore
PMCR_EL0 in the same exact way like the guest was trying to
access
these counters. And then we will also recreate the relevant
host
pmu
events.

Signed-off-by: Jinank Jain <redacted>
Cc: Alexander Graf (AWS) <redacted>
Cc: Marc Zyngier <maz@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Alexandru Elisei <redacted>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
---
 arch/arm64/include/asm/kvm_host.h |  1 +
 arch/arm64/kvm/arm.c              |  1 +
 arch/arm64/kvm/pmu-emul.c         | 10 ++++++++--
 arch/arm64/kvm/pmu.c              | 15 +++++++++++++++
 include/kvm/arm_pmu.h             |  3 +++
 5 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h

b/arch/arm64/include/asm/kvm_host.h
index 7cd7d5c8c4bc..2376ad3c2fc2 100644

--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h

@@ -745,6 +745,7 @@ static inline int

kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
 void kvm_set_pmu_events(u32 set, struct perf_event_attr
*attr);
 void kvm_clr_pmu_events(u32 clr);

+void kvm_vcpu_pmu_restore(struct kvm_vcpu *vcpu);
 void kvm_vcpu_pmu_restore_guest(struct kvm_vcpu *vcpu);
 void kvm_vcpu_pmu_restore_host(struct kvm_vcpu *vcpu);
 #else

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index e720148232a0..c66f6d16ec06 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c

@@ -408,6 +408,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu

*vcpu,
int cpu)
      if (has_vhe())
              kvm_vcpu_load_sysregs_vhe(vcpu);
      kvm_arch_vcpu_load_fp(vcpu);
+     kvm_vcpu_pmu_restore(vcpu);

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.

quoted

      kvm_vcpu_pmu_restore_guest(vcpu);
      if (kvm_arm_is_pvtime_enabled(&vcpu->arch))
              kvm_make_request(KVM_REQ_RECORD_STEAL, vcpu);

diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-

emul.c
index fd167d4f4215..12a40f4b5f0d 100644

--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c

@@ -574,10 +574,16 @@ void kvm_pmu_handle_pmcr(struct kvm_vcpu

*vcpu, u64 val)
              kvm_pmu_disable_counter_mask(vcpu, mask);
      }

-     if (val & ARMV8_PMU_PMCR_C)
+     /*
+      * Cycle counter needs to reset in case of first vcpu
load.
+      */
+     if (val & ARMV8_PMU_PMCR_C ||
!kvm_arm_pmu_v3_restored(vcpu))

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.

Without this we would not be resetting PMU which is required for
creating host perf events. With the patch that you suggested we are
restoring PMCR_EL0 properly but still missing recreation of host
perf
events.

How? The request that gets set on the first vcpu run will call
kvm_pmu_handle_pmcr() -> kvm_pmu_enable_counter_mask() ->
kvm_pmu_create_perf_event(). What are we missing?

I found out what I was missing. I was working with an older kernel
which was missing this upstream patch:

https://lore.kernel.org/lkml/20200124142535.29386-3-eric.auger@redhat.com/ (local)

quoted

And without host perf events, guest would still zeros after live
migration. In my opinion we have two ways to fix it. We can fix it
inside the kernel or let userspace/VMM set those bits before
restarting the guest on the destination machine. What do you think?

I think either you're missing my point above, or I'm completely
missing yours. And I still don't understand why you want to zero the
counters that you have just restored. How does that help?

        M.

--
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