Re: [PATCH v2] perf/x86/amd: support capturing LBR from software events

[Sandipan Das <sandipan.das@amd.com>]

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@fb.com/
>   [1] https://github.com/anakryiko/retsnoop
> 
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

Changes look good to me.

Reviewed-by: Sandipan Das <sandipan.das@amd.com>

> ---
>  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 */