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SubjectRE: [PATCH v8 00/14] Guest LBR Enabling
Date
A polite ping for comments on this version, thanks!

On Tuesday, August 6, 2019 3:16 PM, Wei Wang wrote:
> Last Branch Recording (LBR) is a performance monitor unit (PMU) feature on
> Intel CPUs that captures branch related info. This patch series enables this
> feature to KVM guests.
>
> Each guest can be configured to expose this LBR feature to the guest via
> userspace setting the enabling param in KVM_CAP_X86_GUEST_LBR (patch
> 3).
>
> About the lbr emulation method:
> Since the vcpu get scheduled in, the lbr related msrs are made interceptible.
> This makes guest first access to a lbr related msr always vm-exit to kvm, so
> that kvm can know whether the lbr feature is used during the vcpu time slice.
> The kvm lbr msr handler does the following
> things:
> - create an lbr perf event (task pinned) for the vcpu thread.
> The perf event mainly serves 2 purposes:
> -- follow the host perf scheduling rules to manage the vcpu's usage
> of lbr (e.g. a cpu pinned lbr event could reclaim lbr and thus
> stopping the vcpu's use);
> -- have the host perf do context switching of the lbr state on the
> vcpu thread switching.
> - pass the lbr related msrs through to the guest.
> This enables the following guest accesses to the lbr related msrs
> without vm-exit, as long as the vcpu's lbr event owns the lbr feature.
> A cpu pinned lbr event on the host could come and take over the lbr
> feature via IPI calls. In this case, the pass-through will be
> cancelled (patch 13), and the guest following accesses to the lbr msrs
> will vm-exit to kvm and accesses will be forbidden in the handler.
>
> If the guest doesn't touch any of the lbr related msrs (likely the guest doesn't
> need to run lbr in the near future), the vcpu's lbr perf event will be freed
> (please see patch 12 commit for more details).
>
> * Tests
> Conclusion: the profiling results on the guest are similar to that on the host.
>
> Run: ./perf -b ./test_program
>
> - Test on the host:
> Overhead Command Source Shared Object Source Symbol Target
> Symbol
> 22.35% ftest libc-2.23.so [.] __random [.]
> __random
> 8.20% ftest ftest [.] qux [.] qux
> 5.88% ftest ftest [.] random@plt [.]
> __random
> 5.88% ftest libc-2.23.so [.] __random [.]
> __random_r
> 5.79% ftest ftest [.] main [.]
> random@plt
> 5.60% ftest ftest [.] main [.] foo
> 5.24% ftest libc-2.23.so [.] __random [.] main
> 5.20% ftest libc-2.23.so [.] __random_r [.]
> __random
> 5.00% ftest ftest [.] foo [.] qux
> 4.91% ftest ftest [.] main [.] bar
> 4.83% ftest ftest [.] bar [.] qux
> 4.57% ftest ftest [.] main [.] main
> 4.38% ftest ftest [.] foo [.] main
> 4.13% ftest ftest [.] qux [.] foo
> 3.89% ftest ftest [.] qux [.] bar
> 3.86% ftest ftest [.] bar [.] main
>
> - Test on the guest:
> Overhead Command Source Shaged Object Source Symbol Target
> Symbol
> 22.36% ftest libc-2.23.so [.] random [.] random
> 8.55% ftest ftest [.] qux [.] qux
> 5.79% ftest libc-2.23.so [.] random [.]
> random_r
> 5.64% ftest ftest [.] random@plt [.]
> random
> 5.58% ftest ftest [.] main [.]
> random@plt
> 5.55% ftest ftest [.] main [.] foo
> 5.41% ftest libc-2.23.so [.] random [.] main
> 5.31% ftest libc-2.23.so [.] random_r [.] random
> 5.11% ftest ftest [.] foo [.] qux
> 4.93% ftest ftest [.] main [.] main
> 4.59% ftest ftest [.] qux [.] bar
> 4.49% ftest ftest [.] bar [.] main
> 4.42% ftest ftest [.] bar [.] qux
> 4.16% ftest ftest [.] main [.] bar
> 3.95% ftest ftest [.] qux [.] foo
> 3.79% ftest ftest [.] foo [.] main
> (due to the lib version difference, "random" is equavlent to __random above)
>
> v7->v8 Changelog:
> - Patch 3:
> -- document KVM_CAP_X86_GUEST_LBR in api.txt
> -- make the check of KVM_CAP_X86_GUEST_LBR return the size of
> struct x86_perf_lbr_stack, to let userspace do a compatibility
> check.
> - Patch 7:
> -- support perf scheduler to not assign a counter for the perf event
> that has PERF_EV_CAP_NO_COUNTER set (rather than skipping the
> perf
> scheduler). This allows the scheduler to detect lbr usage conflicts
> via get_event_constraints, and lower priority events will finally
> fail to use lbr.
> -- define X86_PMC_IDX_NA as "-1", which represents a never assigned
> counter id. There are other places that use "-1", but could be
> updated to use the new macro in another patch series.
> - Patch 8:
> -- move the event->owner assignment into perf_event_alloc to have it
> set before event_init is called. Please see this patch's commit for
> reasons.
> - Patch 9:
> -- use "exclude_host" and "is_kernel_event" to decide if the lbr event
> is used for the vcpu lbr emulation, which doesn't need a counter,
> and removes the usage of the previous new perf_event_create API.
> -- remove the unused attr fields.
> - Patch 10:
> -- set a hardware reserved bit (bit 62 of LBR_SELECT) to reg->config
> for the vcpu lbr emulation event. This makes the config different
> from other host lbr event, so that they don't share the lbr.
> Please see the comments in the patch for the reasons why they
> shouldn't share.
> - Patch 12:
> -- disable interrupt and check if the vcpu lbr event owns the lbr
> feature before kvm writing to the lbr related msr. This avoids kvm
> updating the lbr msrs after lbr has been reclaimed by other events
> via ipi.
> -- remove arch v4 related support.
> - Patch 13:
> -- double check if the vcpu lbr event owns the lbr feature before
> vm-entry into the guest. The lbr pass-through will be cancelled if
> lbr feature has been reclaimed by a cpu pinned lbr event.
>
> Previous:
> https://lkml.kernel.org/r/1562548999-37095-1-git-send-email-wei.w.wang
> @intel.com
>
> Wei Wang (14):
> perf/x86: fix the variable type of the lbr msrs
> perf/x86: add a function to get the addresses of the lbr stack msrs
> KVM/x86: KVM_CAP_X86_GUEST_LBR
> KVM/x86: intel_pmu_lbr_enable
> KVM/x86/vPMU: tweak kvm_pmu_get_msr
> KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to the guest
> perf/x86: support to create a perf event without counter allocation
> perf/core: set the event->owner before event_init
> KVM/x86/vPMU: APIs to create/free lbr perf event for a vcpu thread
> perf/x86/lbr: don't share lbr for the vcpu usage case
> perf/x86: save/restore LBR_SELECT on vcpu switching
> KVM/x86/lbr: lbr emulation
> KVM/x86/vPMU: check the lbr feature before entering guest
> KVM/x86: remove the common handling of the debugctl msr
>
> Documentation/virt/kvm/api.txt | 26 +++
> arch/x86/events/core.c | 36 ++-
> arch/x86/events/intel/core.c | 3 +
> arch/x86/events/intel/lbr.c | 95 +++++++-
> arch/x86/events/perf_event.h | 6 +-
> arch/x86/include/asm/kvm_host.h | 5 +
> arch/x86/include/asm/perf_event.h | 17 ++
> arch/x86/kvm/cpuid.c | 2 +-
> arch/x86/kvm/pmu.c | 24 +-
> arch/x86/kvm/pmu.h | 11 +-
> arch/x86/kvm/pmu_amd.c | 7 +-
> arch/x86/kvm/vmx/pmu_intel.c | 476
> +++++++++++++++++++++++++++++++++++++-
> arch/x86/kvm/vmx/vmx.c | 4 +-
> arch/x86/kvm/vmx/vmx.h | 2 +
> arch/x86/kvm/x86.c | 47 ++--
> include/linux/perf_event.h | 18 ++
> include/uapi/linux/kvm.h | 1 +
> kernel/events/core.c | 19 +-
> 18 files changed, 738 insertions(+), 61 deletions(-)
>
> --
> 2.7.4

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