On 3/28/2024 7:03 PM, Andrii Nakryiko wrote: > [0] added ability to capture LBR (Last Branch Records) on Intel CPUs > from inside BPF program at pretty much any arbitrary point. This is > extremely useful capability that allows to figure out otherwise > hard-to-debug problems, because LBR is now available based on some > application-defined conditions, not just hardware-supported events. > > retsnoop ([1]) is one such tool that takes a huge advantage of this > functionality and has proved to be an extremely useful tool in > practice. > > Now, AMD Zen4 CPUs got support for similar LBR functionality, but > necessary wiring inside the kernel is not yet setup. This patch seeks to > rectify this and follows a similar approach to the original patch [0] > for Intel CPUs. > > Given LBR can be set up to capture any indirect jumps, it's critical to > minimize indirect jumps on the way to requesting LBR from BPF program, > so we split amd_pmu_lbr_disable_all() into a wrapper with some generic > conditions vs always-inlined __amd_pmu_lbr_disable() called directly > from BPF subsystem (through perf_snapshot_branch_stack static call). > > Now that it's possible to capture LBR on AMD CPU from BPF at arbitrary > point, there is no reason to artificially limit this feature to sampling > events. So corresponding check is removed. AFAIU, there is no > correctness implications of doing this (and it was possible to bypass > this check by just setting perf_event's sample_period to 1 anyways, so > it doesn't guard all that much). > > This was tested on AMD Bergamo CPU and worked well when utilized from > the aforementioned retsnoop tool. > > [0] https://lore.kernel.org/bpf/20210910183352.3151445-2-songliubraving@xxxxxx/ > [1] https://github.com/anakryiko/retsnoop > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> Changes look good to me. Reviewed-by: Sandipan Das <sandipan.das@xxxxxxx> > --- > arch/x86/events/amd/core.c | 29 ++++++++++++++++++++++++++++- > arch/x86/events/amd/lbr.c | 11 +---------- > arch/x86/events/perf_event.h | 11 +++++++++++ > 3 files changed, 40 insertions(+), 11 deletions(-) > > diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c > index aec16e581f5b..88f6d0701342 100644 > --- a/arch/x86/events/amd/core.c > +++ b/arch/x86/events/amd/core.c > @@ -618,7 +618,7 @@ static void amd_pmu_cpu_dead(int cpu) > } > } > > -static inline void amd_pmu_set_global_ctl(u64 ctl) > +static __always_inline void amd_pmu_set_global_ctl(u64 ctl) > { > wrmsrl(MSR_AMD64_PERF_CNTR_GLOBAL_CTL, ctl); > } > @@ -878,6 +878,29 @@ static int amd_pmu_handle_irq(struct pt_regs *regs) > return amd_pmu_adjust_nmi_window(handled); > } > > +static int amd_pmu_v2_snapshot_branch_stack(struct perf_branch_entry *entries, unsigned int cnt) > +{ > + struct cpu_hw_events *cpuc; > + unsigned long flags; > + > + /* must not have branches... */ > + local_irq_save(flags); > + amd_pmu_core_disable_all(); > + __amd_pmu_lbr_disable(); > + /* ... until here */ > + > + cpuc = this_cpu_ptr(&cpu_hw_events); > + > + amd_pmu_lbr_read(); > + cnt = min_t(unsigned int, cnt, x86_pmu.lbr_nr); > + memcpy(entries, cpuc->lbr_entries, sizeof(struct perf_branch_entry) * cnt); > + > + amd_pmu_v2_enable_all(0); > + local_irq_restore(flags); > + > + return cnt; > +} > + > static int amd_pmu_v2_handle_irq(struct pt_regs *regs) > { > struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events); > @@ -1414,6 +1437,10 @@ static int __init amd_core_pmu_init(void) > static_call_update(amd_pmu_branch_reset, amd_pmu_lbr_reset); > static_call_update(amd_pmu_branch_add, amd_pmu_lbr_add); > static_call_update(amd_pmu_branch_del, amd_pmu_lbr_del); > + > + /* only support branch_stack snapshot on perfmon v2 */ > + if (x86_pmu.handle_irq == amd_pmu_v2_handle_irq) > + static_call_update(perf_snapshot_branch_stack, amd_pmu_v2_snapshot_branch_stack); > } else if (!amd_brs_init()) { > /* > * BRS requires special event constraints and flushing on ctxsw. > diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c > index 4a1e600314d5..75920f895d67 100644 > --- a/arch/x86/events/amd/lbr.c > +++ b/arch/x86/events/amd/lbr.c > @@ -310,10 +310,6 @@ int amd_pmu_lbr_hw_config(struct perf_event *event) > { > int ret = 0; > > - /* LBR is not recommended in counting mode */ > - if (!is_sampling_event(event)) > - return -EINVAL; > - > ret = amd_pmu_lbr_setup_filter(event); > if (!ret) > event->attach_state |= PERF_ATTACH_SCHED_CB; > @@ -412,16 +408,11 @@ void amd_pmu_lbr_enable_all(void) > void amd_pmu_lbr_disable_all(void) > { > struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events); > - u64 dbg_ctl, dbg_extn_cfg; > > if (!cpuc->lbr_users || !x86_pmu.lbr_nr) > return; > > - rdmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg); > - rdmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl); > - > - wrmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg & ~DBG_EXTN_CFG_LBRV2EN); > - wrmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl & ~DEBUGCTLMSR_FREEZE_LBRS_ON_PMI); > + __amd_pmu_lbr_disable(); > } > > __init int amd_pmu_lbr_init(void) > diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h > index fb56518356ec..4dddf0a7e81e 100644 > --- a/arch/x86/events/perf_event.h > +++ b/arch/x86/events/perf_event.h > @@ -1329,6 +1329,17 @@ void amd_pmu_lbr_enable_all(void); > void amd_pmu_lbr_disable_all(void); > int amd_pmu_lbr_hw_config(struct perf_event *event); > > +static __always_inline void __amd_pmu_lbr_disable(void) > +{ > + u64 dbg_ctl, dbg_extn_cfg; > + > + rdmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg); > + rdmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl); > + > + wrmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg & ~DBG_EXTN_CFG_LBRV2EN); > + wrmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl & ~DEBUGCTLMSR_FREEZE_LBRS_ON_PMI); > +} > + > #ifdef CONFIG_PERF_EVENTS_AMD_BRS > > #define AMD_FAM19H_BRS_EVENT 0xc4 /* RETIRED_TAKEN_BRANCH_INSTRUCTIONS */
