brs.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Implement support for AMD Fam19h Branch Sampling feature
 * Based on specifications published in AMD PPR Fam19 Model 01
 *
 * Copyright 2021 Google LLC
 * Contributed by Stephane Eranian <[email protected]>
 */
#include <linux/kernel.h>
#include <linux/jump_label.h>
#include <asm/msr.h>
#include <asm/cpufeature.h>

#include "../perf_event.h"

#define BRS_POISON	0xFFFFFFFFFFFFFFFEULL /* mark limit of valid entries */

/* Debug Extension Configuration register layout */
union amd_debug_extn_cfg {
	__u64 val;
	struct {
		__u64	rsvd0:2,  /* reserved */
			brsmen:1, /* branch sample enable */
			rsvd4_3:2,/* reserved - must be 0x3 */
			vb:1,     /* valid branches recorded */
			rsvd2:10, /* reserved */
			msroff:4, /* index of next entry to write */
			rsvd3:4,  /* reserved */
			pmc:3,    /* #PMC holding the sampling event */
			rsvd4:37; /* reserved */
	};
};

static inline unsigned int brs_from(int idx)
{
	return MSR_AMD_SAMP_BR_FROM + 2 * idx;
}

static inline unsigned int brs_to(int idx)
{
	return MSR_AMD_SAMP_BR_FROM + 2 * idx + 1;
}

static __always_inline void set_debug_extn_cfg(u64 val)
{
	/* bits[4:3] must always be set to 11b */
	native_wrmsrq(MSR_AMD_DBG_EXTN_CFG, val | 3ULL << 3);
}

static __always_inline u64 get_debug_extn_cfg(void)
{
	return native_rdmsrq(MSR_AMD_DBG_EXTN_CFG);
}

static bool __init amd_brs_detect(void)
{
	if (!cpu_feature_enabled(X86_FEATURE_BRS))
		return false;

	switch (boot_cpu_data.x86) {
	case 0x19: /* AMD Fam19h (Zen3) */
		x86_pmu.lbr_nr = 16;

		/* No hardware filtering supported */
		x86_pmu.lbr_sel_map = NULL;
		x86_pmu.lbr_sel_mask = 0;
		break;
	default:
		return false;
	}

	return true;
}

/*
 * Current BRS implementation does not support branch type or privilege level
 * filtering. Therefore, this function simply enforces these limitations. No need for
 * a br_sel_map. Software filtering is not supported because it would not correlate well
 * with a sampling period.
 */
static int amd_brs_setup_filter(struct perf_event *event)
{
	u64 type = event->attr.branch_sample_type;

	/* No BRS support */
	if (!x86_pmu.lbr_nr)
		return -EOPNOTSUPP;

	/* Can only capture all branches, i.e., no filtering */
	if ((type & ~PERF_SAMPLE_BRANCH_PLM_ALL) != PERF_SAMPLE_BRANCH_ANY)
		return -EINVAL;

	return 0;
}

static inline int amd_is_brs_event(struct perf_event *e)
{
	return (e->hw.config & AMD64_RAW_EVENT_MASK) == AMD_FAM19H_BRS_EVENT;
}

int amd_brs_hw_config(struct perf_event *event)
{
	int ret = 0;

	/*
	 * Due to interrupt holding, BRS is not recommended in
	 * counting mode.
	 */
	if (!is_sampling_event(event))
		return -EINVAL;

	/*
	 * Due to the way BRS operates by holding the interrupt until
	 * lbr_nr entries have been captured, it does not make sense
	 * to allow sampling on BRS with an event that does not match
	 * what BRS is capturing, i.e., retired taken branches.
	 * Otherwise the correlation with the event's period is even
	 * more loose:
	 *
	 * With retired taken branch:
	 *   Effective P = P + 16 + X
	 * With any other event:
	 *   Effective P = P + Y + X
	 *
	 * Where X is the number of taken branches due to interrupt
	 * skid. Skid is large.
	 *
	 * Where Y is the occurrences of the event while BRS is
	 * capturing the lbr_nr entries.
	 *
	 * By using retired taken branches, we limit the impact on the
	 * Y variable. We know it cannot be more than the depth of
	 * BRS.
	 */
	if (!amd_is_brs_event(event))
		return -EINVAL;

	/*
	 * BRS implementation does not work with frequency mode
	 * reprogramming of the period.
	 */
	if (event->attr.freq)
		return -EINVAL;
	/*
	 * The kernel subtracts BRS depth from period, so it must
	 * be big enough.
	 */
	if (event->attr.sample_period <= x86_pmu.lbr_nr)
		return -EINVAL;

	/*
	 * Check if we can allow PERF_SAMPLE_BRANCH_STACK
	 */
	ret = amd_brs_setup_filter(event);

	/* only set in case of success */
	if (!ret)
		event->hw.flags |= PERF_X86_EVENT_AMD_BRS;

	return ret;
}

/* tos = top of stack, i.e., last valid entry written */
static inline int amd_brs_get_tos(union amd_debug_extn_cfg *cfg)
{
	/*
	 * msroff: index of next entry to write so top-of-stack is one off
	 * if BRS is full then msroff is set back to 0.
	 */
	return (cfg->msroff ? cfg->msroff : x86_pmu.lbr_nr) - 1;
}

/*
 * make sure we have a sane BRS offset to begin with
 * especially with kexec
 */
void amd_brs_reset(void)
{
	if (!cpu_feature_enabled(X86_FEATURE_BRS))
		return;

	/*
	 * Reset config
	 */
	set_debug_extn_cfg(0);

	/*
	 * Mark first entry as poisoned
	 */
	wrmsrq(brs_to(0), BRS_POISON);
}

int __init amd_brs_init(void)
{
	if (!amd_brs_detect())
		return -EOPNOTSUPP;

	pr_cont("%d-deep BRS, ", x86_pmu.lbr_nr);

	return 0;
}

void amd_brs_enable(void)
{
	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
	union amd_debug_extn_cfg cfg;

	/* Activate only on first user */
	if (++cpuc->brs_active > 1)
		return;

	cfg.val    = 0; /* reset all fields */
	cfg.brsmen = 1; /* enable branch sampling */

	/* Set enable bit */
	set_debug_extn_cfg(cfg.val);
}

void amd_brs_enable_all(void)
{
	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
	if (cpuc->lbr_users)
		amd_brs_enable();
}

void amd_brs_disable(void)
{
	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
	union amd_debug_extn_cfg cfg;

	/* Check if active (could be disabled via x86_pmu_disable_all()) */
	if (!cpuc->brs_active)
		return;

	/* Only disable for last user */
	if (--cpuc->brs_active)
		return;

	/*
	 * Clear the brsmen bit but preserve the others as they contain
	 * useful state such as vb and msroff
	 */
	cfg.val = get_debug_extn_cfg();

	/*
	 * When coming in on interrupt and BRS is full, then hw will have
	 * already stopped BRS, no need to issue wrmsr again
	 */
	if (cfg.brsmen) {
		cfg.brsmen = 0;
		set_debug_extn_cfg(cfg.val);
	}
}

void amd_brs_disable_all(void)
{
	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
	if (cpuc->lbr_users)
		amd_brs_disable();
}

static bool amd_brs_match_plm(struct perf_event *event, u64 to)
{
	int type = event->attr.branch_sample_type;
	int plm_k = PERF_SAMPLE_BRANCH_KERNEL | PERF_SAMPLE_BRANCH_HV;
	int plm_u = PERF_SAMPLE_BRANCH_USER;

	if (!(type & plm_k) && kernel_ip(to))
		return 0;

	if (!(type & plm_u) && !kernel_ip(to))
		return 0;

	return 1;
}

/*
 * Caller must ensure amd_brs_inuse() is true before calling
 * return:
 */
void amd_brs_drain(void)
{
	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
	struct perf_event *event = cpuc->events[0];
	struct perf_branch_entry *br = cpuc->lbr_entries;
	union amd_debug_extn_cfg cfg;
	u32 i, nr = 0, num, tos, start;
	u32 shift = 64 - boot_cpu_data.x86_virt_bits;

	/*
	 * BRS event forced on PMC0,
	 * so check if there is an event.
	 * It is possible to have lbr_users > 0 but the event
	 * not yet scheduled due to long latency PMU irq
	 */
	if (!event)
		goto empty;

	cfg.val = get_debug_extn_cfg();

	/* Sanity check [0-x86_pmu.lbr_nr] */
	if (WARN_ON_ONCE(cfg.msroff >= x86_pmu.lbr_nr))
		goto empty;

	/* No valid branch */
	if (cfg.vb == 0)
		goto empty;

	/*
	 * msr.off points to next entry to be written
	 * tos = most recent entry index = msr.off - 1
	 * BRS register buffer saturates, so we know we have
	 * start < tos and that we have to read from start to tos
	 */
	start = 0;
	tos = amd_brs_get_tos(&cfg);

	num = tos - start + 1;

	/*
	 * BRS is only one pass (saturation) from MSROFF to depth-1
	 * MSROFF wraps to zero when buffer is full
	 */
	for (i = 0; i < num; i++) {
		u32 brs_idx = tos - i;
		u64 from, to;

		rdmsrq(brs_to(brs_idx), to);

		/* Entry does not belong to us (as marked by kernel) */
		if (to == BRS_POISON)
			break;

		/*
		 * Sign-extend SAMP_BR_TO to 64 bits, bits 61-63 are reserved.
		 * Necessary to generate proper virtual addresses suitable for
		 * symbolization
		 */
		to = (u64)(((s64)to << shift) >> shift);

		if (!amd_brs_match_plm(event, to))
			continue;

		rdmsrq(brs_from(brs_idx), from);

		perf_clear_branch_entry_bitfields(br+nr);

		br[nr].from = from;
		br[nr].to   = to;

		nr++;
	}
empty:
	/* Record number of sampled branches */
	cpuc->lbr_stack.nr = nr;
}

/*
 * Poison most recent entry to prevent reuse by next task
 * required because BRS entry are not tagged by PID
 */
static void amd_brs_poison_buffer(void)
{
	union amd_debug_extn_cfg cfg;
	unsigned int idx;

	/* Get current state */
	cfg.val = get_debug_extn_cfg();

	/* idx is most recently written entry */
	idx = amd_brs_get_tos(&cfg);

	/* Poison target of entry */
	wrmsrq(brs_to(idx), BRS_POISON);
}

/*
 * On context switch in, we need to make sure no samples from previous user
 * are left in the BRS.
 *
 * On ctxswin, sched_in = true, called after the PMU has started
 * On ctxswout, sched_in = false, called before the PMU is stopped
 */
void amd_pmu_brs_sched_task(struct perf_event_pmu_context *pmu_ctx,
			    struct task_struct *task, bool sched_in)
{
	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);

	/* no active users */
	if (!cpuc->lbr_users)
		return;

	/*
	 * On context switch in, we need to ensure we do not use entries
	 * from previous BRS user on that CPU, so we poison the buffer as
	 * a faster way compared to resetting all entries.
	 */
	if (sched_in)
		amd_brs_poison_buffer();
}

/*
 * called from ACPI processor_idle.c or acpi_pad.c
 * with interrupts disabled
 */
void noinstr perf_amd_brs_lopwr_cb(bool lopwr_in)
{
	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
	union amd_debug_extn_cfg cfg;

	/*
	 * on mwait in, we may end up in non C0 state.
	 * we must disable branch sampling to avoid holding the NMI
	 * for too long. We disable it in hardware but we
	 * keep the state in cpuc, so we can re-enable.
	 *
	 * The hardware will deliver the NMI if needed when brsmen cleared
	 */
	if (cpuc->brs_active) {
		cfg.val = get_debug_extn_cfg();
		cfg.brsmen = !lopwr_in;
		set_debug_extn_cfg(cfg.val);
	}
}

DEFINE_STATIC_CALL_NULL(perf_lopwr_cb, perf_amd_brs_lopwr_cb);
EXPORT_STATIC_CALL_TRAMP_GPL(perf_lopwr_cb);

void __init amd_brs_lopwr_init(void)
{
	static_call_update(perf_lopwr_cb, perf_amd_brs_lopwr_cb);
}