Re: [PATCH v13 00/10] Guest Last Branch Recording Enabling (KVM part)

["Xu, Like" <like.xu@intel.com>]

Are there no interested reviewers or users?

Just a kindly ping.

On 2020/7/26 23:32, Like Xu wrote:
> Hi Paolo,
>
> Please review this new version for the Kernel 5.9 release, and
> Sean may not review them as he said in the previous email
> https://lore.kernel.org/kvm/20200710162819.GF1749@linux.intel.com/
>
> You may cherry-pick the perf patches "3cb9d5464c1c..e1ad1ac2deb8"
> from the branch "tip/perf/core" of scm/linux/kernel/git/tip/tip.git
> as PeterZ said in the previous email
> https://lore.kernel.org/kvm/20200703075646.GJ117543@hirez.programming.kicks-ass.net/
>
> We may also apply the qemu-devel patch to the upstream qemu and try
> the QEMU command lines with '-cpu host' or '-cpu host,pmu=true,lbr=true'.
>
> The following error will be gone forever with the patchset:
>
>    $ 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'
>
> Please check more details in each commit and feel free to test.
>
> v12->v13 Changelog:
> - remove perf patches since they're queued in the tip/perf/core;
> - add a minor patch to refactor MSR_IA32_DEBUGCTLMSR set/get handler;
> - add a minor patch to expose vmx_set_intercept_for_msr();
> - add a minor patch to initialize perf_capabilities in the intel_pmu_init();
> - spilt the big patch to three pieces (0004-0006) for better understanding and review
> - make the LBR_FMT exposure patch as the last step to enable guest LBR;
>
> Previous:
> https://lore.kernel.org/kvm/20200613080958.132489-1-like.xu@linux.intel.com/
>
> ---
>
> 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.
>
> 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 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 (10):
>    KVM: x86: Move common set/get handler of MSR_IA32_DEBUGCTLMSR to VMX
>    KVM: x86/vmx: Make vmx_set_intercept_for_msr() non-static and expose it
>    KVM: vmx/pmu: Initialize vcpu perf_capabilities once in intel_pmu_init()
>    KVM: vmx/pmu: Clear PMU_CAP_LBR_FMT when guest LBR is disabled
>    KVM: vmx/pmu: Create a guest LBR event when vcpu sets DEBUGCTLMSR_LBR
>    KVM: vmx/pmu: Pass-through LBR msrs to 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: Expose LBR_FMT in the MSR_IA32_PERF_CAPABILITIES
>    KVM: vmx/pmu: Release guest LBR event via lazy release mechanism
>
>   arch/x86/kvm/pmu.c              |  12 +-
>   arch/x86/kvm/pmu.h              |   5 +
>   arch/x86/kvm/vmx/capabilities.h |  22 ++-
>   arch/x86/kvm/vmx/pmu_intel.c    | 296 +++++++++++++++++++++++++++++++-
>   arch/x86/kvm/vmx/vmx.c          |  44 ++++-
>   arch/x86/kvm/vmx/vmx.h          |  28 +++
>   arch/x86/kvm/x86.c              |  15 +-
>   7 files changed, 395 insertions(+), 27 deletions(-)
>