Re: [PATCH V7 6/6] arm64/perf: Enable branch stack events via FEAT_BRBE

Anshuman Khandual
2023-01-19T02:50:29Z

On 1/12/23 22:21, Mark Rutland wrote:
> On Thu, Jan 05, 2023 at 08:40:39AM +0530, Anshuman Khandual wrote:
>> This enables branch stack sampling events in ARMV8 PMU, via an architecture
>> feature FEAT_BRBE aka branch record buffer extension. This defines required
>> branch helper functions pmuv8pmu_branch_XXXXX() and the implementation here
>> is wrapped with a new config option CONFIG_ARM64_BRBE.
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> arch/arm64/Kconfig | 11 +
>> arch/arm64/include/asm/perf_event.h | 9 +
>> arch/arm64/kernel/Makefile | 1 +
>> arch/arm64/kernel/brbe.c | 512 ++++++++++++++++++++++++++++
>> arch/arm64/kernel/brbe.h | 257 ++++++++++++++
>> 5 files changed, 790 insertions(+)
>> create mode 100644 arch/arm64/kernel/brbe.c
>> create mode 100644 arch/arm64/kernel/brbe.h
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 03934808b2ed..915b12709a46 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -1363,6 +1363,17 @@ config HW_PERF_EVENTS
>> def_bool y
>> depends on ARM_PMU
>>
>> +config ARM64_BRBE
>> + bool "Enable support for Branch Record Buffer Extension (BRBE)"
>> + depends on PERF_EVENTS && ARM64 && ARM_PMU
>> + default y
>> + help
>> + Enable perf support for Branch Record Buffer Extension (BRBE) which
>> + records all branches taken in an execution path. This supports some
>> + branch types and privilege based filtering. It captured additional
>> + relevant information such as cycle count, misprediction and branch
>> + type, branch privilege level etc.
>> +
>> # Supported by clang >= 7.0 or GCC >= 12.0.0
>> config CC_HAVE_SHADOW_CALL_STACK
>> def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)
>> diff --git a/arch/arm64/include/asm/perf_event.h b/arch/arm64/include/asm/perf_event.h
>> index a038902d6874..cf2e88c7b707 100644
>> --- a/arch/arm64/include/asm/perf_event.h
>> +++ b/arch/arm64/include/asm/perf_event.h
>> @@ -277,6 +277,14 @@ struct pmu_hw_events;
>> struct arm_pmu;
>> struct perf_event;
>>
>> +#ifdef CONFIG_ARM64_BRBE
>> +void armv8pmu_branch_read(struct pmu_hw_events *cpuc, struct perf_event *event);
>> +bool armv8pmu_branch_valid(struct perf_event *event);
>> +void armv8pmu_branch_enable(struct perf_event *event);
>> +void armv8pmu_branch_disable(struct perf_event *event);
>> +void armv8pmu_branch_probe(struct arm_pmu *arm_pmu);
>> +void armv8pmu_branch_reset(void);
>> +#else
>> static inline void armv8pmu_branch_read(struct pmu_hw_events *cpuc, struct perf_event *event) { }
>> static inline bool armv8pmu_branch_valid(struct perf_event *event) { return false; }
>> static inline void armv8pmu_branch_enable(struct perf_event *event) { }
>> @@ -284,3 +292,4 @@ static inline void armv8pmu_branch_disable(struct perf_event *event) { }
>> static inline void armv8pmu_branch_probe(struct arm_pmu *arm_pmu) { }
>> static inline void armv8pmu_branch_reset(void) { }
>> #endif
>> +#endif
>> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
>> index ceba6792f5b3..6ee7ccb61621 100644
>> --- a/arch/arm64/kernel/Makefile
>> +++ b/arch/arm64/kernel/Makefile
>> @@ -46,6 +46,7 @@ obj-$(CONFIG_MODULES) += module.o
>> obj-$(CONFIG_ARM64_MODULE_PLTS) += module-plts.o
>> obj-$(CONFIG_PERF_EVENTS) += perf_regs.o perf_callchain.o
>> obj-$(CONFIG_HW_PERF_EVENTS) += perf_event.o
>> +obj-$(CONFIG_ARM64_BRBE) += brbe.o
>> obj-$(CONFIG_HAVE_HW_BREAKPOINT) += hw_breakpoint.o
>> obj-$(CONFIG_CPU_PM) += sleep.o suspend.o
>> obj-$(CONFIG_CPU_IDLE) += cpuidle.o
>> diff --git a/arch/arm64/kernel/brbe.c b/arch/arm64/kernel/brbe.c
>> new file mode 100644
>> index 000000000000..cd03d3531e04
>> --- /dev/null
>> +++ b/arch/arm64/kernel/brbe.c
>> @@ -0,0 +1,512 @@
>> +// SPDX-License-Identifier: GPL-2.0-only
>> +/*
>> + * Branch Record Buffer Extension Driver.
>> + *
>> + * Copyright (C) 2022 ARM Limited
>> + *
>> + * Author: Anshuman Khandual <anshuman.khandual@arm.com>
>> + */
>> +#include "brbe.h"
>> +
>> +static bool valid_brbe_nr(int brbe_nr)
>> +{
>> + return brbe_nr == BRBIDR0_EL1_NUMREC_8 ||
>> + brbe_nr == BRBIDR0_EL1_NUMREC_16 ||
>> + brbe_nr == BRBIDR0_EL1_NUMREC_32 ||
>> + brbe_nr == BRBIDR0_EL1_NUMREC_64;
>> +}
>> +
>> +static bool valid_brbe_cc(int brbe_cc)
>> +{
>> + return brbe_cc == BRBIDR0_EL1_CC_20_BIT;
>> +}
>> +
>> +static bool valid_brbe_format(int brbe_format)
>> +{
>> + return brbe_format == BRBIDR0_EL1_FORMAT_0;
>> +}
>> +
>> +static bool valid_brbe_version(int brbe_version)
>> +{
>> + return brbe_version == ID_AA64DFR0_EL1_BRBE_IMP ||
>> + brbe_version == ID_AA64DFR0_EL1_BRBE_BRBE_V1P1;
>> +}
>> +
>> +static void select_brbe_bank(int bank)
>> +{
>> + static int brbe_current_bank = BRBE_BANK_IDX_INVALID;
>
> This is a per-cpu peroperty, so I don't understand how this can safely be
> stored in a static variable. If this is necessary it needs to go in a per-cpu
> variable, but I suspect we don't actually need it.

You are right, we dont need it.

>
>> + u64 brbfcr;
>> +
>> + if (brbe_current_bank == bank)
>> + return;
>
> It looks like this is just for the same of optimizing redundant changes when
> armv8pmu_branch_read() iterates over the records?

Right, it is.

>
> It'd be simpler to have armv8pmu_branch_read() iterate over each bank, then
> within that iterate over each record within that bank.

Sure, will drop this optimization completely. I will split the iteration into two
separate loops, one each for bank 0 and other for bank 1.

>
>> + WARN_ON(bank > BRBE_BANK_IDX_1);
>> + brbfcr = read_sysreg_s(SYS_BRBFCR_EL1);
>> + brbfcr &= ~BRBFCR_EL1_BANK_MASK;
>> + brbfcr |= ((bank << BRBFCR_EL1_BANK_SHIFT) & BRBFCR_EL1_BANK_MASK);
>
> You can use SYS_FIELD_PREP() for this:

Sure, will do.

>
> brbfcr &= ~BRBFCR_EL1_BANK_MASK;
> brbfcr |= SYS_FIELD_PREP(BRBFCR_EL1, BANK, bank);
>
> Please use FIELD_PREP for this.

Done.

>
>> + write_sysreg_s(brbfcr, SYS_BRBFCR_EL1);
>> + isb();
>> + brbe_current_bank = bank;
>> +}
>> +
>> +static void select_brbe_bank_index(int buffer_idx)
>> +{
>> + switch (buffer_idx) {
>> + case BRBE_BANK0_IDX_MIN ... BRBE_BANK0_IDX_MAX:
>> + select_brbe_bank(BRBE_BANK_IDX_0);
>> + break;
>> + case BRBE_BANK1_IDX_MIN ... BRBE_BANK1_IDX_MAX:
>> + select_brbe_bank(BRBE_BANK_IDX_1);
>> + break;
>> + default:
>> + pr_warn("unsupported BRBE index\n");
>
> It would be worth logging the specific index in case we ever have to debug
> this. It's probably worth also making this a WARN_ONCE() or WARN_RATELIMITED().

This function will not be required once individual loops based read for each
BRBE bank is implemented, thus reducing number of times select_brbe_bank()
gets called i.e just two times once for bank 0 and other for bank 1.

>
>> + }
>> +}
>> +
>> +static const char branch_filter_error_msg[] = "branch filter not supported";
>> +
>> +bool armv8pmu_branch_valid(struct perf_event *event)
>> +{
>> + u64 branch_type = event->attr.branch_sample_type;
>> +
>> + /*
>> + * If the event does not have at least one of the privilege
>> + * branch filters as in PERF_SAMPLE_BRANCH_PLM_ALL, the core
>> + * perf will adjust its value based on perf event's existing
>> + * privilege level via attr.exclude_[user|kernel|hv].
>> + *
>> + * As event->attr.branch_sample_type might have been changed
>> + * when the event reaches here, it is not possible to figure
>> + * out whether the event originally had HV privilege request
>> + * or got added via the core perf. Just report this situation
>> + * once and continue ignoring if there are other instances.
>> + */
>> + if ((branch_type & PERF_SAMPLE_BRANCH_HV) && !is_kernel_in_hyp_mode())
>> + pr_warn_once("%s - hypervisor privilege\n", branch_filter_error_msg);
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_ABORT_TX) {
>> + pr_warn_once("%s - aborted transaction\n", branch_filter_error_msg);
>> + return false;
>> + }
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_NO_TX) {
>> + pr_warn_once("%s - no transaction\n", branch_filter_error_msg);
>> + return false;
>> + }
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_IN_TX) {
>> + pr_warn_once("%s - in transaction\n", branch_filter_error_msg);
>> + return false;
>> + }
>> + return true;
>> +}
>
> Is this called when validating user input? If so, NAK to printing anything to a
> higher leval than debug. If there are constraints the user needs to be aware of

You mean pr_debug() based prints ?

> we should expose the relevant information under sysfs, but it seems that these
> are just generic perf options that BRBE doesn't support.

Right, these are generic perf options. As you mentioned, will replace these with
pr_debug() instead.

>
> It would be better to whitelist what we do support rather than blacklisting
> what we don't.

But with a negative list, user would know what is not supported via these pr_debug()
based output when enabled ? But I dont have a strong opinion either way.

>
>> +
>> +static void branch_records_alloc(struct arm_pmu *armpmu)
>> +{
>> + struct pmu_hw_events *events;
>> + int cpu;
>> +
>> + for_each_possible_cpu(cpu) {
>> + events = per_cpu_ptr(armpmu->hw_events, cpu);
>> +
>> + events->branches = kzalloc(sizeof(struct branch_records), GFP_KERNEL);
>> + WARN_ON(!events->branches);
>> + }
>> +}
>
> It would be simpler for this to be a percpu allocation.

Could you please be more specific ? alloc_percpu_gfp() cannot be used here
because 'events->branches' is not a __percpu variable unlike its parent
'events' which is derived from armpmu.

>
> If the allocation fails, we should propogate that error rather than just
> WARNing, and fail probing the PMU.

Sure, will change that.

>
> Also, if the generic allocator fails it will print a warning (unless
> __GFP_NOWARN was used), so we don't need the warning here.

Sure, understood.

>
>> +
>> +static int brbe_attributes_probe(struct arm_pmu *armpmu, u32 brbe)
>> +{
>> + struct brbe_hw_attr *brbe_attr = kzalloc(sizeof(struct brbe_hw_attr), GFP_KERNEL);
>
> Same comments as for the failure path in branch_records_alloc().
>
>> + u64 brbidr = read_sysreg_s(SYS_BRBIDR0_EL1);
>
> Which context is this run in? Unless this is affine to a relevant CPU we can't
> read the sysreg safely, and if we're in a cross-call we cannot allocate memory,
> so this doesn't look right to me.

Called from smp_call_function_any() context via __armv8pmu_probe_pmu().

>
> I suspect CONFIG_DEBUG_ATOMIC_SLEEP=y and/or CONFIG_PROVE_LOCKING=y will complain here.

Right, it does. Remember dropping pr_info() during BRBE probe for the exact same
reason here but did not realize we will run into the same problem again.

>
> Please follow the approach of armv8pmu_probe_pmu(), where we use a probe_info
> structure that the callee can fill with information. Then we can do the
> allocation in the main thread from a non-atomic context.

Right, will do that. The only problem being 'struct brbe_hw_attr' which will not be
visible in the main thread, might need an abstraction function to do the allocation
in BRBE implementation. Regardless, a successful BRBE in the preceding function can
be ascertained from armpmu->has_branch_stack().

>
>> +
>> + WARN_ON(!brbe_attr);
>> + armpmu->private = brbe_attr;
>> +
>> + brbe_attr->brbe_version = brbe;
>> + brbe_attr->brbe_format = brbe_fetch_format(brbidr);
>> + brbe_attr->brbe_cc = brbe_fetch_cc_bits(brbidr);
>> + brbe_attr->brbe_nr = brbe_fetch_numrec(brbidr);
>
> As a minor thing, could we please s/fetch/get/ ? To me, 'fetch' sounds like a
> memory operation, and elsewhere we use 'get' for this sort of getter function.

Sure, but shall we change fetch as get for entire BRBE implementation (where ever
there is a determination of field from a register value) or just the above function ?
Default, will change all places.

>
>> +
>> + if (!valid_brbe_version(brbe_attr->brbe_version) ||
>> + !valid_brbe_format(brbe_attr->brbe_format) ||
>> + !valid_brbe_cc(brbe_attr->brbe_cc) ||
>> + !valid_brbe_nr(brbe_attr->brbe_nr))
>> + return -EOPNOTSUPP;
>> +
>> + return 0;
>> +}
>> +
>> +void armv8pmu_branch_probe(struct arm_pmu *armpmu)
>> +{
>> + u64 aa64dfr0 = read_sysreg_s(SYS_ID_AA64DFR0_EL1);
>> + u32 brbe;
>> +
>> + brbe = cpuid_feature_extract_unsigned_field(aa64dfr0, ID_AA64DFR0_EL1_BRBE_SHIFT);
>> + if (!brbe)
>> + return;
>> +
>> + if (brbe_attributes_probe(armpmu, brbe))
>> + return;
>> +
>> + branch_records_alloc(armpmu);
>> + armpmu->features |= ARM_PMU_BRANCH_STACK;
>> +}
>> +
>> +static u64 branch_type_to_brbfcr(int branch_type)
>> +{
>> + u64 brbfcr = 0;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_ANY) {
>> + brbfcr |= BRBFCR_EL1_BRANCH_FILTERS;
>> + return brbfcr;
>> + }
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_ANY_CALL) {
>> + brbfcr |= BRBFCR_EL1_INDCALL;
>> + brbfcr |= BRBFCR_EL1_DIRCALL;
>> + }
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_ANY_RETURN)
>> + brbfcr |= BRBFCR_EL1_RTN;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_IND_CALL)
>> + brbfcr |= BRBFCR_EL1_INDCALL;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_COND)
>> + brbfcr |= BRBFCR_EL1_CONDDIR;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_IND_JUMP)
>> + brbfcr |= BRBFCR_EL1_INDIRECT;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_CALL)
>> + brbfcr |= BRBFCR_EL1_DIRCALL;
>> +
>> + return brbfcr;
>> +}
>> +
>> +static u64 branch_type_to_brbcr(int branch_type)
>> +{
>> + u64 brbcr = (BRBCR_EL1_FZP | BRBCR_EL1_DEFAULT_TS);
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_USER)
>> + brbcr |= BRBCR_EL1_E0BRE;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_KERNEL)
>> + brbcr |= BRBCR_EL1_E1BRE;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_HV) {
>> + if (is_kernel_in_hyp_mode())
>> + brbcr |= BRBCR_EL1_E1BRE;
>> + }
>
> I assume that in that case we're actually writing to BRBCR_EL2, and this is
> actually the E2BRE bit, which is at the same position? If so, I think that's
> worth a comment above the USER/KERNEL/HV bits here.

That is right, will add a comment.

>
> How do the BRB* control registers work with E2H? Is BRBCR_EL1 rewritten to
> BRBCR_EL2 by the hardware?

Right, that is my understanding as well.

With FEAT_VHE and HCR_EL2.E2H = 1, access to BRBCR_EL1 at EL2, accesses BRBCR_EL2
without FEAT_VHE or HCR_EL2.E2H = 0, access to BRBCR_EL1 at EL2, accesses BRBCR_EL1

>
>> +
>> + if (!(branch_type & PERF_SAMPLE_BRANCH_NO_CYCLES))
>> + brbcr |= BRBCR_EL1_CC;
>> +
>> + if (!(branch_type & PERF_SAMPLE_BRANCH_NO_FLAGS))
>> + brbcr |= BRBCR_EL1_MPRED;
>> +
>> + /*
>> + * The exception and exception return branches could be
>> + * captured, irrespective of the perf event's privilege.
>> + * If the perf event does not have enough privilege for
>> + * a given exception level, then addresses which falls
>> + * under that exception level will be reported as zero
>> + * for the captured branch record, creating source only
>> + * or target only records.
>> + */
>> + if (branch_type & PERF_SAMPLE_BRANCH_ANY) {
>> + brbcr |= BRBCR_EL1_EXCEPTION;
>> + brbcr |= BRBCR_EL1_ERTN;
>> + }
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_ANY_CALL)
>> + brbcr |= BRBCR_EL1_EXCEPTION;
>> +
>> + if (branch_type & PERF_SAMPLE_BRANCH_ANY_RETURN)
>> + brbcr |= BRBCR_EL1_ERTN;
>> +
>> + return brbcr & BRBCR_EL1_DEFAULT_CONFIG;
>> +}
>> +
>> +void armv8pmu_branch_enable(struct perf_event *event)
>> +{
>> + u64 branch_type = event->attr.branch_sample_type;
>> + u64 brbfcr, brbcr;
>> +
>> + brbfcr = read_sysreg_s(SYS_BRBFCR_EL1);
>> + brbfcr &= ~BRBFCR_EL1_DEFAULT_CONFIG;
>> + brbfcr |= branch_type_to_brbfcr(branch_type);
>> + write_sysreg_s(brbfcr, SYS_BRBFCR_EL1);
>> + isb();
>> +
>> + brbcr = read_sysreg_s(SYS_BRBCR_EL1);
>> + brbcr &= ~BRBCR_EL1_DEFAULT_CONFIG;
>> + brbcr |= branch_type_to_brbcr(branch_type);
>> + write_sysreg_s(brbcr, SYS_BRBCR_EL1);
>> + isb();
>> + armv8pmu_branch_reset();
>> +}
>> +
>> +void armv8pmu_branch_disable(struct perf_event *event)
>> +{
>> + u64 brbfcr = read_sysreg_s(SYS_BRBFCR_EL1);
>> + u64 brbcr = read_sysreg_s(SYS_BRBCR_EL1);
>> +
>> + brbcr &= ~(BRBCR_EL1_E0BRE | BRBCR_EL1_E1BRE);
>> + brbfcr |= BRBFCR_EL1_PAUSED;
>> + write_sysreg_s(brbcr, SYS_BRBCR_EL1);
>> + write_sysreg_s(brbfcr, SYS_BRBFCR_EL1);
>> + isb();
>> +}
>> +
>> +static int brbe_fetch_perf_type(u64 brbinf, bool *new_branch_type)
>
> It's a bit confusing to return the type and new_type fields in this way.
>
> I think this would be clearer as a setter function, even if that results in it
> being a bit longer, since it keeps all the type and new_type relationships in
> one place and has a single path for returning the value:

Makes sense.

>
> static void brbe_set_perf_entry_type(struct perf_branch_entry *entry,
> u64 brbinf)
> {
> int brbe_type = brbe_fetch_type(brbinf);
>
> switch (brbe_type) {
> case BRBINF_EL1_TYPE_UNCOND_DIR;
> entry->type = PERF_BR_UNCOND;
> break;
> ...
> case BRBINF_EL1_TYPE_DEBUG_HALT;
> entry->type = PERF_BR_EXTEND_ABI;
> entry->new_type = PERF_BR_ARM64_DEBUG_HALT;
> break;
> ...
> default:
> ...
> }
> }
>
> ... and in theory that makes it easier to propogate an error in future if we
> want to.

Sure, will convert this function into brbe_set_perf_entry_type() as suggested.

>
>> +{
>> + int brbe_type = brbe_fetch_type(brbinf);
>> + *new_branch_type = false;
>> +
>> + switch (brbe_type) {
>> + case BRBINF_EL1_TYPE_UNCOND_DIR:
>> + return PERF_BR_UNCOND;
>> + case BRBINF_EL1_TYPE_INDIR:
>> + return PERF_BR_IND;
>> + case BRBINF_EL1_TYPE_DIR_LINK:
>> + return PERF_BR_CALL;
>> + case BRBINF_EL1_TYPE_INDIR_LINK:
>> + return PERF_BR_IND_CALL;
>> + case BRBINF_EL1_TYPE_RET_SUB:
>> + return PERF_BR_RET;
>> + case BRBINF_EL1_TYPE_COND_DIR:
>> + return PERF_BR_COND;
>> + case BRBINF_EL1_TYPE_CALL:
>> + return PERF_BR_CALL;
>> + case BRBINF_EL1_TYPE_TRAP:
>> + return PERF_BR_SYSCALL;
>> + case BRBINF_EL1_TYPE_RET_EXCPT:
>> + return PERF_BR_ERET;
>> + case BRBINF_EL1_TYPE_IRQ:
>> + return PERF_BR_IRQ;
>> + case BRBINF_EL1_TYPE_DEBUG_HALT:
>> + *new_branch_type = true;
>> + return PERF_BR_ARM64_DEBUG_HALT;
>> + case BRBINF_EL1_TYPE_SERROR:
>> + return PERF_BR_SERROR;
>> + case BRBINF_EL1_TYPE_INST_DEBUG:
>> + *new_branch_type = true;
>> + return PERF_BR_ARM64_DEBUG_INST;
>> + case BRBINF_EL1_TYPE_DATA_DEBUG:
>> + *new_branch_type = true;
>> + return PERF_BR_ARM64_DEBUG_DATA;
>> + case BRBINF_EL1_TYPE_ALGN_FAULT:
>> + *new_branch_type = true;
>> + return PERF_BR_NEW_FAULT_ALGN;
>> + case BRBINF_EL1_TYPE_INST_FAULT:
>> + *new_branch_type = true;
>> + return PERF_BR_NEW_FAULT_INST;
>> + case BRBINF_EL1_TYPE_DATA_FAULT:
>> + *new_branch_type = true;
>> + return PERF_BR_NEW_FAULT_DATA;
>> + case BRBINF_EL1_TYPE_FIQ:
>> + *new_branch_type = true;
>> + return PERF_BR_ARM64_FIQ;
>> + case BRBINF_EL1_TYPE_DEBUG_EXIT:
>> + *new_branch_type = true;
>> + return PERF_BR_ARM64_DEBUG_EXIT;
>> + default:
>> + pr_warn("unknown branch type captured\n");
>> + return PERF_BR_UNKNOWN;
>
> It would be worth logging the specific value in case we ever have to debug
> this. This should also be marked as _ratelimited or _once.

Sure, will replace with a pr_warn_once() printing 'branch_type'.

>
>> + }
>> +}
>> +
>> +static int brbe_fetch_perf_priv(u64 brbinf)
>> +{
>> + int brbe_el = brbe_fetch_el(brbinf);
>> +
>> + switch (brbe_el) {
>> + case BRBINF_EL1_EL_EL0:
>> + return PERF_BR_PRIV_USER;
>> + case BRBINF_EL1_EL_EL1:
>> + return PERF_BR_PRIV_KERNEL;
>> + case BRBINF_EL1_EL_EL2:
>> + if (is_kernel_in_hyp_mode())
>> + return PERF_BR_PRIV_KERNEL;
>> + return PERF_BR_PRIV_HV;
>> + default:
>> + pr_warn("unknown branch privilege captured\n");
>> + return PERF_BR_PRIV_UNKNOWN;
>
> It would be worth logging the specific value in case we ever have to debug
> this. This should also be marked as _ratelimited or _once.

Sure, will replace with a pr_warn_once() printing 'brbe_el.

>
>> + }
>> +}
>> +
>> +static void capture_brbe_flags(struct pmu_hw_events *cpuc, struct perf_event *event,
>> + u64 brbinf, u64 brbcr, int idx)
>> +{
>> + struct perf_branch_entry *entry = &cpuc->branches->branch_entries[idx];
>> + bool new_branch_type;
>> + int branch_type;
>> +
>> + if (branch_sample_type(event)) {
>> + branch_type = brbe_fetch_perf_type(brbinf, &new_branch_type);
>> + if (new_branch_type) {
>> + entry->type = PERF_BR_EXTEND_ABI;
>> + entry->new_type = branch_type;
>> + } else {
>> + entry->type = branch_type;
>> + }
>> + }
>
> With the suggestions bove, this would become:
>
> if (branch_sample_type(event))
> brbe_set_perf_entry_type(entry, brbinf);

That's right, will change.

>
>> + if (!branch_sample_no_cycles(event)) {
>> + WARN_ON_ONCE(!(brbcr & BRBCR_EL1_CC));
>> + entry->cycles = brbe_fetch_cycles(brbinf);
>> + }
>> +
>> + if (!branch_sample_no_flags(event)) {
>> + /*
>> + * BRBINF_LASTFAILED does not indicate whether last transaction
>> + * got failed or aborted during the current branch record itself.
>> + * Rather, this indicates that all the branch records which were
>> + * in transaction until the curret branch record have failed. So
>> + * the entire BRBE buffer needs to be processed later on to find
>> + * all branch records which might have failed.
>> + */
>
> This is quite difficult to follow.
>
> I took in the ARM ARM, and it looks like this is all about TME transactions
> (which Linux doesn't currently support). Per ARM DDI 0487I.a, page D15-5506:
>
> | R_GVCJH
> | When an entire transaction is executed in a BRBE Non-prohibited region and
> | the transaction fails or is canceled then BRBFCR_EL1.LASTFAILED is set to
> | 1.
>
> | R_KBSZM
> | When a Branch record is generated, other than through the injection
> | mechanism, the value of BRBFCR_EL1.LASTFAILED is copied to the LASTFAILED
> | field in the Branch record and BRBFCR_EL1.LASTFAILED is set to 0.
>
> | I_JBPHS
> | When a transaction fails or is canceled, Branch records generated in the
> | transaction are not removed from the Branch record buffer.
>
> I think what this is saying is:
>
> /*
> * BRBINFx_EL1.LASTFAILED indicates that a TME transaction failed (or
> * was cancelled) prior to this record, and some number of records
> * prior to this one may have been generated during an attempt to
> * execute the transaction.
> *
> * We will remove such entries later in process_branch_aborts().
> */
>
> Is that right?

Right, will update the comment here.

>
>> +
>> + /*
>> + * All these information (i.e transaction state and mispredicts)
>> + * are not available for target only branch records.
>> + */
>> + if (!brbe_target(brbinf)) {
>
> Could we rename these heleprs for clarity, e.g.
> brbe_record_is_{target_only,source_only,complete}()

Sure, will do.

>
> With that, it would also be clearer to have:
>
> /*
> * These fields only exist for complete and source-only records.
> */
> if (brbe_record_is_complete(brbinf) ||
> brbe_record_is_source_only()) {
>
> ... and explicilty match the cases we care about[

Sure, will invert the check and update the comment here.

>
>
>> + WARN_ON_ONCE(!(brbcr & BRBCR_EL1_MPRED));
>
> Huh? Why does the value of BRBCR matter here?

This is just a code hardening measure here. Before recording branch record
cycles or its flags, ensure BRBCR_EL1 was configured correctly to produce
these additional information along with the branch records.

>
>> + entry->mispred = brbe_fetch_mispredict(brbinf);
>> + entry->predicted = !entry->mispred;
>> + entry->in_tx = brbe_fetch_in_tx(brbinf);
>> + }
>> + }
>> +
>> + if (branch_sample_priv(event)) {
>> + /*
>> + * All these information (i.e branch privilege level) are not
>> + * available for source only branch records.
>> + */
>> + if (!brbe_source(brbinf))
>> + entry->priv = brbe_fetch_perf_priv(brbinf);
>
> Same style comment as above.

Sure, will do.

>
>> + }
>> +}
>> +
>> +/*
>> + * A branch record with BRBINF_EL1.LASTFAILED set, implies that all
>> + * preceding consecutive branch records, that were in a transaction
>> + * (i.e their BRBINF_EL1.TX set) have been aborted.
>> + *
>> + * Similarly BRBFCR_EL1.LASTFAILED set, indicate that all preceding
>> + * consecutive branch records upto the last record, which were in a
>> + * transaction (i.e their BRBINF_EL1.TX set) have been aborted.
>> + *
>> + * --------------------------------- -------------------
>> + * | 00 | BRBSRC | BRBTGT | BRBINF | | TX = 1 | LF = 0 | [TX success]
>> + * --------------------------------- -------------------
>> + * | 01 | BRBSRC | BRBTGT | BRBINF | | TX = 1 | LF = 0 | [TX success]
>> + * --------------------------------- -------------------
>> + * | 02 | BRBSRC | BRBTGT | BRBINF | | TX = 0 | LF = 0 |
>> + * --------------------------------- -------------------
>> + * | 03 | BRBSRC | BRBTGT | BRBINF | | TX = 1 | LF = 0 | [TX failed]
>> + * --------------------------------- -------------------
>> + * | 04 | BRBSRC | BRBTGT | BRBINF | | TX = 1 | LF = 0 | [TX failed]
>> + * --------------------------------- -------------------
>> + * | 05 | BRBSRC | BRBTGT | BRBINF | | TX = 0 | LF = 1 |
>> + * --------------------------------- -------------------
>> + * | .. | BRBSRC | BRBTGT | BRBINF | | TX = 0 | LF = 0 |
>> + * --------------------------------- -------------------
>> + * | 61 | BRBSRC | BRBTGT | BRBINF | | TX = 1 | LF = 0 | [TX failed]
>> + * --------------------------------- -------------------
>> + * | 62 | BRBSRC | BRBTGT | BRBINF | | TX = 1 | LF = 0 | [TX failed]
>> + * --------------------------------- -------------------
>> + * | 63 | BRBSRC | BRBTGT | BRBINF | | TX = 1 | LF = 0 | [TX failed]
>> + * --------------------------------- -------------------
>
> Are we guaranteed to have a record between two transactions with TX = 0?

With TX = 0 i.e no transaction was active, indicates normal sequence of branches
creating their own branch records. How can there be a transaction with TX = 0 ?
Could you please be more specific here ?

>
> AFAICT you could have a sequence where a TCOMMIT is immediately followed by a
> TSTART, and IIUC in that case you could have back-to-back records for distinct
> transactions all with TX = 1, where the first transaction could be commited,
> and the second might fail/cancel.
>
> ... or do TCOMMIT/TCANCEL/TSTART get handled specially?

I guess these are micro-architectural implementation details which unfortunately
BRBINF_EL1/BRBCR_EL1 specifications do not capture in detail. But all it says is
that upon encountering BRBINF_EL1.LASTFAILED or BRBFCR_EL1.LASTFAILED (just for
the last record) all previous in-transaction branch records (BRBINF_EL1.TX = 1)
should be considered aborted for branch record reporting purpose.

>
>> + *
>> + * BRBFCR_EL1.LASTFAILED == 1
>> + *
>> + * Here BRBFCR_EL1.LASTFAILED failes all those consecutive and also
>> + * in transaction branches near the end of the BRBE buffer.
>> + */
>> +static void process_branch_aborts(struct pmu_hw_events *cpuc)
>> +{
>> + struct brbe_hw_attr *brbe_attr = (struct brbe_hw_attr *)cpuc->percpu_pmu->private;
>> + u64 brbfcr = read_sysreg_s(SYS_BRBFCR_EL1);
>> + bool lastfailed = !!(brbfcr & BRBFCR_EL1_LASTFAILED);
>> + int idx = brbe_attr->brbe_nr - 1;
>> + struct perf_branch_entry *entry;
>> +
>> + do {
>> + entry = &cpuc->branches->branch_entries[idx];
>> + if (entry->in_tx) {
>> + entry->abort = lastfailed;
>> + } else {
>> + lastfailed = entry->abort;
>> + entry->abort = false;
>> + }
>> + } while (idx--, idx >= 0);
>> +}
>> +
>> +void armv8pmu_branch_reset(void)
>> +{
>> + asm volatile(BRB_IALL);
>> + isb();
>> +}
>> +
>> +void armv8pmu_branch_read(struct pmu_hw_events *cpuc, struct perf_event *event)
>> +{
>> + struct brbe_hw_attr *brbe_attr = (struct brbe_hw_attr *)cpuc->percpu_pmu->private;
>> + u64 brbinf, brbfcr, brbcr;
>> + int idx;
>> +
>> + brbcr = read_sysreg_s(SYS_BRBCR_EL1);
>> + brbfcr = read_sysreg_s(SYS_BRBFCR_EL1);
>> +
>> + /* Ensure pause on PMU interrupt is enabled */
>> + WARN_ON_ONCE(!(brbcr & BRBCR_EL1_FZP));
>
> As above, I think this needs commentary in the interrupt handler, since this
> presumably needs us to keep the IRQ asserted until we're done
> reading/manipulating records in the IRQ handler.

The base IRQ handler armv8pmu_handle_irq() is in ARMV8 PMU code inside perf_event.c
which could/should not access BRBE specific details without adding an another new
abstraction function. But I guess adding a comment should be fine.

>
> Do we ever read this outside of the IRQ handler? AFAICT we don't, and that
> makes it seem like some of this is redundant.

>
>> +
>> + /* Save and clear the privilege */
>> + write_sysreg_s(brbcr & ~(BRBCR_EL1_E0BRE | BRBCR_EL1_E1BRE), SYS_BRBCR_EL1);
>
> Why? Later on we restore this, and AFAICT we don't modify it.
>
> If it's paused, why do we care about the privilege?

This disables BRBE completely (not only pause) providing confidence that no
branch record can come in while the existing records are being processed.

>
>> +
>> + /* Pause the buffer */
>> + write_sysreg_s(brbfcr | BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1);
>> + isb();
>
> Why? If we're in the IRQ handler it's already paused, and if we're not in the
> IRQ handler what prevents us racing with an IRQ?

armv8pmu_branch_read() always gets called from an IRQ context only. The point
here is to force a pause (and also disable, as I had explained earlier) before
reading the buffer.

>
>> +
>> + for (idx = 0; idx < brbe_attr->brbe_nr; idx++) {
>> + struct perf_branch_entry *entry = &cpuc->branches->branch_entries[idx];
>> +
>> + select_brbe_bank_index(idx);
>> + brbinf = get_brbinf_reg(idx);
>> + /*
>> + * There are no valid entries anymore on the buffer.
>> + * Abort the branch record processing to save some
>> + * cycles and also reduce the capture/process load
>> + * for the user space as well.
>> + */
>> + if (brbe_invalid(brbinf))
>> + break;
>> +
>> + perf_clear_branch_entry_bitfields(entry);
>> + if (brbe_valid(brbinf)) {
>> + entry->from = get_brbsrc_reg(idx);
>> + entry->to = get_brbtgt_reg(idx);
>> + } else if (brbe_source(brbinf)) {
>> + entry->from = get_brbsrc_reg(idx);
>> + entry->to = 0;
>> + } else if (brbe_target(brbinf)) {
>> + entry->from = 0;
>> + entry->to = get_brbtgt_reg(idx);
>> + }
>> + capture_brbe_flags(cpuc, event, brbinf, brbcr, idx);
>> + }
>> + cpuc->branches->branch_stack.nr = idx;
>> + cpuc->branches->branch_stack.hw_idx = -1ULL;
>> + process_branch_aborts(cpuc);
>> +
>> + /* Restore privilege, enable pause on PMU interrupt */
>> + write_sysreg_s(brbcr | BRBCR_EL1_FZP, SYS_BRBCR_EL1);
>
> Why do we have to save/restore this?

Yes, this guarantees (more so than the paused state) that BRBE remains disabled in
privilege levels that are relevant, while the contents are being read.

>
>> +
>> + /* Unpause the buffer */
>> + write_sysreg_s(brbfcr & ~BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1);
>> + isb();
>> + armv8pmu_branch_reset();
>> +}
>
> Why do we enable it before we reset it?

This is the last opportunity for a clean slate start for BRBE buffer before it is
back recording the branches. Basically helps in ensuring a clean start.

>
> Surely it would make sense to reset it first, and ammortize the cost of the ISB?
>
> That said, as above, do we actually need to pause/unpause it? Or is it already
> paused by virtue of the IRQ?

Yes, it should be paused after an IRQ but it is also enforced before reading along
with privilege level disable. Regardless the buffer needs to be un-paused and also
enabled for required privilege levels before exiting from here.