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Date: Tue, 19 Dec 2023 13:38:07 +0530
From: Sandipan Das <sandipan.das@....com>
To: Stephane Eranian <eranian@...gle.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 12/19/2023 2:13 AM, Stephane Eranian wrote:
> 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.
>
Agreed. Will make it a visible flag in /proc/cpuinfo.
- Sandipan
>> 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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