Date:   Mon, 7 Jun 2021 18:34:08 +0000
From:   "Jain, Jinank" <jinankj@...zon.de>
To:     "maz@...nel.org" <maz@...nel.org>
CC:     "james.morse@....com" <james.morse@....com>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "suzuki.poulose@....com" <suzuki.poulose@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexandru.elisei@....com" <alexandru.elisei@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "will@...nel.org" <will@...nel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "Graf (AWS), Alexander" <graf@...zon.de>
Subject: Re: [PATCH] KVM: arm64: Properly restore PMU state during live-migration

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" <jinankj@...zon.de> wrote:
> > On Thu, 2021-06-03 at 17:03 +0100, Marc Zyngier wrote:
> > > Hi Jinank,
> > > 
> > > On Thu, 03 Jun 2021 12:05:54 +0100,
> > > Jinank Jain <jinankj@...zon.de> 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"?
> > > 
> > > > 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 <jinankj@...zon.de>
> > > > Cc: Alexander Graf (AWS) <graf@...zon.de>
> > > > Cc: Marc Zyngier <maz@...nel.org>
> > > > Cc: James Morse <james.morse@....com>
> > > > Cc: Alexandru Elisei <alexandru.elisei@....com>
> > > > Cc: Suzuki K Poulose <suzuki.poulose@....com>
> > > > Cc: Catalin Marinas <catalin.marinas@....com>
> > > > Cc: Will Deacon <will@...nel.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.
> > > 
> > > >       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/

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



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