Hi Marc. On Mon, 2021-06-07 at 17:35 +0100, Marc Zyngier wrote: > CAUTION: This email originated from outside of the organization. Do > not click links or open attachments unless you can confirm the sender > and know the content is safe. > > > > On Mon, 07 Jun 2021 17:05:01 +0100, > "Jain, Jinank" <jinankj@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> > > > > Cc: Alexander Graf (AWS) <graf@xxxxxxxxx> > > > > Cc: Marc Zyngier <maz@xxxxxxxxxx> > > > > Cc: James Morse <james.morse@xxxxxxx> > > > > Cc: Alexandru Elisei <alexandru.elisei@xxxxxxx> > > > > Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx> > > > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx> > > > > Cc: Will Deacon <will@xxxxxxxxxx> > > > > --- > > > > 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@xxxxxxxxxx/ > > 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. Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879 _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
