Messages in this thread | | | Subject | Re: [PATCH v14 00/11] KVM: x86/pmu: Guest Last Branch Recording Enabling | From | Paolo Bonzini <> | Date | Tue, 2 Feb 2021 13:37:56 +0100 |
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On 01/02/21 06:10, Like Xu wrote: > Hi geniuses, > > Please help review this new version which enables the guest LBR. > > We already upstreamed the guest LBR support in the host perf, please > check more details in each commit and feel free to test and comment. > > QEMU part: https://lore.kernel.org/qemu-devel/20210201045453.240258-1-like.xu@linux.intel.com > kvm-unit-tests: https://lore.kernel.org/kvm/20210201045751.243231-1-like.xu@linux.intel.com > > v13-v14 Changelog: > - Rewrite crud about vcpu->arch.perf_capabilities; > - Add PERF_CAPABILITIES testcases to tools/testing/selftests/kvm; > - Add basic LBR testcases to the kvm-unit-tests (w/ QEMU patches); > - Apply rewritten commit log from Paolo; > - Queued the first patch "KVM: x86: Move common set/get handler ..."; > - Rename 'already_passthrough' to 'msr_passthrough'; > - Check the values of MSR_IA32_PERF_CAPABILITIES early; > - Call kvm_x86_ops.pmu_ops->cleanup() always and drop extra_cleanup; > - Use INTEL_PMC_IDX_FIXED_VLBR directly; > - Fix a bug in the vmx_get_perf_capabilities(); > > Previous: > https://lore.kernel.org/kvm/20210108013704.134985-1-like.xu@linux.intel.com/
Queued, thanks. There were some conflicts with the bus lock detection patches, so I had to tweak a bit the DEBUGCTL MSR handling.
Paolo
> --- > > The last branch recording (LBR) is a performance monitor unit (PMU) > feature on Intel processors that records a running trace of the most > recent branches taken by the processor in the LBR stack. This patch > series is going to enable this feature for plenty of KVM guests. > > with this patch set, the following error will be gone forever and cloud > developers can better understand their programs with less profiling overhead: > > $ perf record -b lbr ${WORKLOAD} > or $ perf record --call-graph lbr ${WORKLOAD} > Error: > cycles: PMU Hardware doesn't support sampling/overflow-interrupts. Try 'perf stat' > > The user space could configure whether it's enabled or not for each > guest via MSR_IA32_PERF_CAPABILITIES msr. As a first step, a guest > could only enable LBR feature if its cpu model is the same as the > host since the LBR feature is still one of model specific features. > > If it's enabled on the guest, the guest LBR driver would accesses the > LBR MSR (including IA32_DEBUGCTLMSR and records MSRs) as host does. > The first guest access on the LBR related MSRs is always interceptible. > The KVM trap would create a special LBR event (called guest LBR event) > which enables the callstack mode and none of hardware counter is assigned. > The host perf would enable and schedule this event as usual. > > Guest's first access to a LBR registers gets trapped to KVM, which > creates a guest LBR perf event. It's a regular LBR perf event which gets > the LBR facility assigned from the perf subsystem. Once that succeeds, > the LBR stack msrs are passed through to the guest for efficient accesses. > However, if another host LBR event comes in and takes over the LBR > facility, the LBR msrs will be made interceptible, and guest following > accesses to the LBR msrs will be trapped and meaningless. > > Because saving/restoring tens of LBR MSRs (e.g. 32 LBR stack entries) in > VMX transition brings too excessive overhead to frequent vmx transition > itself, the guest LBR event would help save/restore the LBR stack msrs > during the context switching with the help of native LBR event callstack > mechanism, including LBR_SELECT msr. > > If the guest no longer accesses the LBR-related MSRs within a scheduling > time slice and the LBR enable bit is unset, vPMU would release its guest > LBR event as a normal event of a unused vPMC and the pass-through > state of the LBR stack msrs would be canceled. > > --- > > LBR testcase: > echo 1 > /proc/sys/kernel/watchdog > echo 25 > /proc/sys/kernel/perf_cpu_time_max_percent > echo 5000 > /proc/sys/kernel/perf_event_max_sample_rate > echo 0 > /proc/sys/kernel/perf_cpu_time_max_percent > perf record -b ./br_instr a > (perf record --call-graph lbr ./br_instr a) > > - Perf report on the host: > Samples: 72K of event 'cycles', Event count (approx.): 72512 > Overhead Command Source Shared Object Source Symbol Target Symbol Basic Block Cycles > 12.12% br_instr br_instr [.] cmp_end [.] lfsr_cond 1 > 11.05% br_instr br_instr [.] lfsr_cond [.] cmp_end 5 > 8.81% br_instr br_instr [.] lfsr_cond [.] cmp_end 4 > 5.04% br_instr br_instr [.] cmp_end [.] lfsr_cond 20 > 4.92% br_instr br_instr [.] lfsr_cond [.] cmp_end 6 > 4.88% br_instr br_instr [.] cmp_end [.] lfsr_cond 6 > 4.58% br_instr br_instr [.] cmp_end [.] lfsr_cond 5 > > - Perf report on the guest: > Samples: 92K of event 'cycles', Event count (approx.): 92544 > Overhead Command Source Shared Object Source Symbol Target Symbol Basic Block Cycles > 12.03% br_instr br_instr [.] cmp_end [.] lfsr_cond 1 > 11.09% br_instr br_instr [.] lfsr_cond [.] cmp_end 5 > 8.57% br_instr br_instr [.] lfsr_cond [.] cmp_end 4 > 5.08% br_instr br_instr [.] lfsr_cond [.] cmp_end 6 > 5.06% br_instr br_instr [.] cmp_end [.] lfsr_cond 20 > 4.87% br_instr br_instr [.] cmp_end [.] lfsr_cond 6 > 4.70% br_instr br_instr [.] cmp_end [.] lfsr_cond 5 > > Conclusion: the profiling results on the guest are similar to that on the host. > > Like Xu (11): > KVM: x86/vmx: Make vmx_set_intercept_for_msr() non-static > KVM: x86/pmu: Set up IA32_PERF_CAPABILITIES if PDCM bit is available > KVM: vmx/pmu: Add PMU_CAP_LBR_FMT check when guest LBR is enabled > KVM: vmx/pmu: Expose DEBUGCTLMSR_LBR in the MSR_IA32_DEBUGCTLMSR > KVM: vmx/pmu: Create a guest LBR event when vcpu sets DEBUGCTLMSR_LBR > KVM: vmx/pmu: Pass-through LBR msrs when the guest LBR event is ACTIVE > KVM: vmx/pmu: Reduce the overhead of LBR pass-through or cancellation > KVM: vmx/pmu: Emulate legacy freezing LBRs on virtual PMI > KVM: vmx/pmu: Release guest LBR event via lazy release mechanism > KVM: vmx/pmu: Expose LBR_FMT in the MSR_IA32_PERF_CAPABILITIES > selftests: kvm/x86: add test for pmu msr MSR_IA32_PERF_CAPABILITIES > > arch/x86/kvm/pmu.c | 8 +- > arch/x86/kvm/pmu.h | 2 + > arch/x86/kvm/vmx/capabilities.h | 19 +- > arch/x86/kvm/vmx/pmu_intel.c | 281 +++++++++++++++++- > arch/x86/kvm/vmx/vmx.c | 55 +++- > arch/x86/kvm/vmx/vmx.h | 28 ++ > arch/x86/kvm/x86.c | 2 +- > tools/testing/selftests/kvm/.gitignore | 1 + > tools/testing/selftests/kvm/Makefile | 1 + > .../selftests/kvm/x86_64/vmx_pmu_msrs_test.c | 149 ++++++++++ > 10 files changed, 524 insertions(+), 22 deletions(-) > create mode 100644 tools/testing/selftests/kvm/x86_64/vmx_pmu_msrs_test.c >
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