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Date: Mon, 18 Dec 2023 12:43:13 -0800
From: Stephane Eranian <eranian@...gle.com>
To: Sandipan Das <sandipan.das@....com>
Cc: linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
x86@...nel.org, peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
mark.rutland@....com, alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
namhyung@...nel.org, adrian.hunter@...el.com, tglx@...utronix.de,
bp@...en8.de, irogers@...gle.com, mario.limonciello@....com,
ravi.bangoria@....com, ananth.narayan@....com
Subject: Re: [PATCH 1/3] perf/x86/amd/lbr: Use freeze based on availability
On Sun, Dec 17, 2023 at 9:26 AM Sandipan Das <sandipan.das@....com> wrote:
>
> Currently, it is assumed that LBR Freeze is supported on all processors
> which have CPUID leaf 0x80000022[EAX] bit 1 set. This is incorrect as
> the feature availability is additionally dependent on CPUID leaf
> 0x80000022[EAX] bit 2 being set which may not be set for all Zen 4
> processors. Define a new feature bit for LBR and PMC freeze and set the
> freeze enable bit (FLBRI) in DebugCtl (MSR 0x1d9) conditionally.
Is this new feature bit visible to users?
I think it is useful to know whether or not LBR freeze is supported.
Imagine I want to do kernel FDO, then the user-only LBR trick to freeze LBR
does not work and I need actual LBR freeze support.
Thanks.
> It should still be possible to use LBR without freeze for profile-guided
> optimization of user programs by using an user-only branch filter during
> profiling. When the user-only filter is enabled, branches are no longer
> recorded after the transition to CPL 0 upon PMI arrival. When branch
> entries are read in the PMI handler, the branch stack does not change.
>
> E.g.
>
> $ perf record -j any,u -e ex_ret_brn_tkn ./workload
>
> Fixes: ca5b7c0d9621 ("perf/x86/amd/lbr: Add LbrExtV2 branch record support")
> Signed-off-by: Sandipan Das <sandipan.das@....com>
> Cc: stable@...r.kernel.org
> ---
> arch/x86/events/amd/core.c | 4 ++--
> arch/x86/events/amd/lbr.c | 16 ++++++++++------
> arch/x86/include/asm/cpufeatures.h | 2 +-
> arch/x86/kernel/cpu/scattered.c | 1 +
> 4 files changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
> index 4ee6390b45c9..ffdfaee08b08 100644
> --- a/arch/x86/events/amd/core.c
> +++ b/arch/x86/events/amd/core.c
> @@ -905,8 +905,8 @@ static int amd_pmu_v2_handle_irq(struct pt_regs *regs)
> if (!status)
> goto done;
>
> - /* Read branch records before unfreezing */
> - if (status & GLOBAL_STATUS_LBRS_FROZEN) {
> + /* Read branch records */
> + if (x86_pmu.lbr_nr) {
> amd_pmu_lbr_read();
> status &= ~GLOBAL_STATUS_LBRS_FROZEN;
> }
> diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
> index eb31f850841a..110e34c59643 100644
> --- a/arch/x86/events/amd/lbr.c
> +++ b/arch/x86/events/amd/lbr.c
> @@ -400,10 +400,12 @@ void amd_pmu_lbr_enable_all(void)
> wrmsrl(MSR_AMD64_LBR_SELECT, lbr_select);
> }
>
> - rdmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl);
> - rdmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg);
> + if (cpu_feature_enabled(X86_FEATURE_AMD_LBR_PMC_FREEZE)) {
> + rdmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl);
> + wrmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl | DEBUGCTLMSR_FREEZE_LBRS_ON_PMI);
> + }
>
> - wrmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl | DEBUGCTLMSR_FREEZE_LBRS_ON_PMI);
> + rdmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg);
> wrmsrl(MSR_AMD_DBG_EXTN_CFG, dbg_extn_cfg | DBG_EXTN_CFG_LBRV2EN);
> }
>
> @@ -416,10 +418,12 @@ void amd_pmu_lbr_disable_all(void)
> 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);
> +
> + if (cpu_feature_enabled(X86_FEATURE_AMD_LBR_PMC_FREEZE)) {
> + rdmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl);
> + wrmsrl(MSR_IA32_DEBUGCTLMSR, dbg_ctl & ~DEBUGCTLMSR_FREEZE_LBRS_ON_PMI);
> + }
> }
>
> __init int amd_pmu_lbr_init(void)
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 4af140cf5719..9790e906d5e5 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -97,7 +97,7 @@
> #define X86_FEATURE_SYSENTER32 ( 3*32+15) /* "" sysenter in IA32 userspace */
> #define X86_FEATURE_REP_GOOD ( 3*32+16) /* REP microcode works well */
> #define X86_FEATURE_AMD_LBR_V2 ( 3*32+17) /* AMD Last Branch Record Extension Version 2 */
> -/* FREE, was #define X86_FEATURE_LFENCE_RDTSC ( 3*32+18) "" LFENCE synchronizes RDTSC */
> +#define X86_FEATURE_AMD_LBR_PMC_FREEZE ( 3*32+18) /* "" AMD LBR and PMC Freeze */
> #define X86_FEATURE_ACC_POWER ( 3*32+19) /* AMD Accumulated Power Mechanism */
> #define X86_FEATURE_NOPL ( 3*32+20) /* The NOPL (0F 1F) instructions */
> #define X86_FEATURE_ALWAYS ( 3*32+21) /* "" Always-present feature */
> diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
> index 0dad49a09b7a..a515328d9d7d 100644
> --- a/arch/x86/kernel/cpu/scattered.c
> +++ b/arch/x86/kernel/cpu/scattered.c
> @@ -49,6 +49,7 @@ static const struct cpuid_bit cpuid_bits[] = {
> { X86_FEATURE_BMEC, CPUID_EBX, 3, 0x80000020, 0 },
> { X86_FEATURE_PERFMON_V2, CPUID_EAX, 0, 0x80000022, 0 },
> { X86_FEATURE_AMD_LBR_V2, CPUID_EAX, 1, 0x80000022, 0 },
> + { X86_FEATURE_AMD_LBR_PMC_FREEZE, CPUID_EAX, 2, 0x80000022, 0 },
> { 0, 0, 0, 0, 0 }
> };
>
> --
> 2.34.1
>
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