[PATCH V2 3/6] powerpc/perf: Add AUX buffer management to capture HTM trace data

[Athira Rajeev <atrajeev@linux.ibm.com>]

Implement support for auxiliary (AUX) ring buffers in the HTM PMU driver.
This enables high-volume trace data to be streamed directly into a perf
AUX buffer for deferred post-processing by the perf tool.

Introduce the PMU data structures htm_pmu_buf and htm_pmu_ctx, and
implement the core lifecycle hooks:

  htm_setup_aux():  Allocates per-CPU context and records the AUX buffer
                    base, head, and size for the active trace window.
  htm_free_aux():   Releases the PMU-private tracking allocations.

The perf core AUX allocator (rb_alloc_aux) may return a page array with
physical fragmentation gaps.  H_HTM_OP_DUMP_DATA operates on raw physical
addresses and requires a strictly contiguous region; crossing a gap would
cause silent memory corruption.

To prevent this, opt into PERF_PMU_CAP_AUX_NO_SG | PERF_PMU_CAP_AUX_PREFER_LARGE
to request large, physically contiguous allocations, and perform a
page-by-page physical continuity scan in htm_event_read() before each
H_HTM_OP_DUMP_DATA call.  The scan verifies that
virt_to_phys(page[n]) + PAGE_SIZE == virt_to_phys(page[n+1]) and
breaks the loop on the first discontinuity, ensuring the dump to the
verified safe window.

Update htm_event_read() to set event->count to 1 when data is present
in the buffer and 0 when the stream is exhausted.  This binary flag is
used by the perf tool drain hook to decide whether another read pass is
needed before closing the event.  It does not represent an instruction
or cycle count; the actual trace records are decoded in userspace.

HTM target identity and tracing state are kept in event->pmu_private
(htm_target_id).  AUX-private state holds only buffer metadata and
dump progress; htm_target_id.tracing_active remains the source for
tracing state.

Signed-off-by: Athira Rajeev <atrajeev@linux.ibm.com>
---
Changes in V2:
- Opted into PERF_PMU_CAP_AUX_NO_SG | PERF_PMU_CAP_AUX_PREFER_LARGE to
  request physically contiguous allocations, which H_HTM_OP_DUMP_DATA
  requires.  V1 didn't handle no contiguity requirement.
- Added a page-by-page physical-continuity scan in htm_event_read()
  before every H_HTM_OP_DUMP_DATA call.  The scan verifies that
  virt_to_phys(page[n]) + PAGE_SIZE == virt_to_phys(page[n+1]) and
  stops at the first gap, bounding the dump to a verified safe window.
  V1 had no such guard and could silently corrupt memory across gaps.
- event->count is now set to 1 when trace data is present and 0 when
  the stream is exhausted.  The perf tool uses this as a drain signal.
  V1 relied on a separate arch_record__collect_final_data callback loop
  with explicit evlist__enable cycling; this version doesn't need
  explicit evlist__enable

 arch/powerpc/perf/htm-perf.c | 216 ++++++++++++++++++++++++++++++++++-
 1 file changed, 215 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/perf/htm-perf.c b/arch/powerpc/perf/htm-perf.c
index 01c6bd2104cf..edfca63e1e9b 100644
--- a/arch/powerpc/perf/htm-perf.c
+++ b/arch/powerpc/perf/htm-perf.c
@@ -95,6 +95,23 @@ static inline void parse_htm_config(u64 config, struct htm_config *cfg)
 	cfg->coreindexonchip = (config >> 20) & 0xff;
 }
 
+struct htm_pmu_buf {
+	int     nr_pages;
+	bool    snapshot;
+	void    *base;
+	void	**pages;
+	u64	head;
+	u64     size;
+	int     collect_htm_trace;
+	u64	trace_records;
+};
+
+struct htm_pmu_ctx {
+	struct perf_output_handle handle;
+};
+
+static DEFINE_PER_CPU(struct htm_pmu_ctx, htm_pmu_ctx);
+
 /*
  * Check the return code for H_HTM hcall.
  * Return 1 if either H_PARTIAL or H_SUCCESS is returned.
@@ -372,8 +389,202 @@ static void htm_event_del(struct perf_event *event, int flags)
 	/* pmu_private freed by event->destroy = reset_htm_active */
 }
 
+static int htm_dump_sample_data(struct perf_event *event)
+{
+	struct htm_pmu_ctx *htm_ctx = this_cpu_ptr(&htm_pmu_ctx);
+	struct htm_target_id *target = event->pmu_private;
+	struct htm_pmu_buf *aux_buf;
+	struct htm_config cfg = target->cfg;
+	u64 chunk_size, dump_offset, page_index, page_offset;
+	u64 max_contiguous_bytes, expected_phys, scan_index, actual_phys;
+	u64 hypervisor_target_phys;
+	void *target_page_virt;
+	int ret = 0, retries = 0;
+	long rc;
+
+	/* Start AUX transaction session framework */
+	aux_buf = perf_aux_output_begin(&htm_ctx->handle, event);
+	if (!aux_buf)
+		return 0;
+
+	if (!aux_buf->collect_htm_trace) {
+		perf_aux_output_end(&htm_ctx->handle, 0);
+		return 0;
+	}
+
+	if (target->tracing_active == HTM_TRACING_ACTIVE) {
+		htm_event_stop(event, 0);
+		if (target->tracing_active == HTM_TRACING_ACTIVE) {
+			perf_aux_output_end(&htm_ctx->handle, 0);
+			return -EAGAIN;
+		}
+	}
+
+	/* Derive the exact target destination point directly out of active ring pointers */
+	dump_offset = htm_ctx->handle.head & (aux_buf->size - 1);
+	page_index = dump_offset >> PAGE_SHIFT;
+	page_offset = dump_offset & (PAGE_SIZE - 1);
+
+	/*
+	 * Assess constraints regarding space remaining across the mapping
+	 * context boundary
+	 */
+	chunk_size = htm_ctx->handle.size;
+	chunk_size &= PAGE_MASK;
+
+	if (chunk_size > (aux_buf->size - dump_offset))
+		chunk_size = aux_buf->size - dump_offset;
+
+	/*
+	 * HTM driver uses these capabilities:
+	 * PERF_PMU_CAP_AUX_NO_SG | PERF_PMU_CAP_AUX_PREFER_LARGE
+	 * the core perf ring-buffer allocator (rb_alloc_aux) tries to allocate
+	 * physically contiguous page block. If not available, it tries to allocate
+	 * largest possible contiguous block.
+	 *
+	 * Example: If we ask perf for 1024 pages (64MB), the kernel executes a loop
+	 * inside rb_alloc_aux() to fulfill that request using the buddy allocator. it
+	 * always tries to grab the largest possible contiguous block of memory it can
+	 * find first, then takes the next largest, and repeats until request is
+	 * completely filled. Here while writing to aux buffer, to eliminate any
+	 * virtual or physical boundary overruns, check for the page boundary.
+	 *
+	 * Dynamically scan forward page-by-page from our active page_index to
+	 * calculate the absolute boundary limit of this current physically
+	 * contiguous block chunk. Prevents hypervisor macro overruns across
+	 * asymmetrical fragmentation gaps.
+	 */
+	max_contiguous_bytes = PAGE_SIZE - page_offset;
+	scan_index = page_index + 1;
+	expected_phys = (u64)virt_to_phys(aux_buf->pages[page_index]) + PAGE_SIZE;
+
+	while (scan_index < aux_buf->nr_pages && max_contiguous_bytes < chunk_size) {
+		actual_phys = (u64)virt_to_phys(aux_buf->pages[scan_index]);
+
+		if (actual_phys != expected_phys)
+			break; /* Intersected a fragmentation boundary block link! */
+
+		max_contiguous_bytes += PAGE_SIZE;
+		expected_phys += PAGE_SIZE;
+		scan_index++;
+	}
+
+	/* Bound transfer length tightly within the validated contiguous window */
+	if (chunk_size > max_contiguous_bytes)
+		chunk_size = max_contiguous_bytes;
+
+	if (!chunk_size) {
+		aux_buf->collect_htm_trace = 0;
+		perf_aux_output_end(&htm_ctx->handle, 0);
+		return 0;
+	}
+
+	/*
+	 * Compute the precise base target address using
+	 * localized page offset rules
+	 */
+	target_page_virt = aux_buf->pages[page_index];
+	hypervisor_target_phys = (u64)virt_to_phys(target_page_virt) + page_offset;
+
+	do {
+		/*
+		 * Invoke H_HTM call with:
+		 * - operation as htm dump (H_HTM_OP_DUMP_DATA)
+		 * - last three values are address, size and offset
+		 */
+		rc = htm_hcall_wrapper(htmflags, cfg.nodeindex, cfg.nodalchipindex,
+				cfg.coreindexonchip, cfg.htmtype, H_HTM_OP_DUMP_DATA,
+				hypervisor_target_phys, chunk_size, aux_buf->head);
+		ret = htm_return_check(rc);
+	} while (ret == -EBUSY && ++retries < MAX_RETRIES);
+
+	if (ret > 0) {
+		aux_buf->head += chunk_size;
+		aux_buf->trace_records++;
+		perf_aux_output_end(&htm_ctx->handle, chunk_size);
+		return ret;
+	}
+
+	aux_buf->collect_htm_trace = 0;
+	perf_aux_output_end(&htm_ctx->handle, 0);
+	return ret;
+}
+
 static void htm_event_read(struct perf_event *event)
 {
+	int ret;
+
+	/*
+	 * Update event->count as a binary indicator:
+	 * 1 if data was dumped into the
+	 * AUX buffer, 0 otherwise. Actual trace record
+	 * decoding is left to userspace.
+	 */
+	ret = htm_dump_sample_data(event);
+	if (ret <= 0)
+		local64_set(&event->count, 0);
+	else
+		local64_set(&event->count, 1);
+}
+
+/*
+ * Set up pmu-private data structures for an AUX area
+ * **pages contains the aux buffer allocated for this event
+ * for the corresponding cpu. rb_alloc_aux uses "alloc_pages_node"
+ * and returns pointer to each page address.
+ * PMU capabilities: PERF_PMU_CAP_AUX_NO_SG | PERF_PMU_CAP_AUX_PREFER_LARGE
+ * to try get closest possible physically contiguous page blocks.
+ *
+ * The aux private data structure ie, "struct htm_pmu_buf" mainly
+ * saves
+ * - buf->base: aux buffer base address
+ * - buf->head: offset from base address where data will be written to.
+ * - buf->size: Size of allocated memory
+ */
+static void *htm_setup_aux(struct perf_event *event, void **pages,
+		int nr_pages, bool snapshot)
+{
+	int cpu = event->cpu;
+	struct htm_pmu_buf *buf;
+
+	if (!nr_pages)
+		return NULL;
+
+	if (cpu == -1)
+		cpu = raw_smp_processor_id();
+
+	buf = kzalloc_node(sizeof(*buf), GFP_KERNEL, cpu_to_node(cpu));
+	if (!buf)
+		return NULL;
+
+	buf->nr_pages = nr_pages;
+	buf->snapshot = snapshot;
+	buf->size = (u64)nr_pages << PAGE_SHIFT;
+	buf->pages = pages;
+
+	buf->base = pages[0];
+	if (!buf->base) {
+		kfree(buf);
+		return NULL;
+	}
+
+	buf->collect_htm_trace = 1;
+	buf->trace_records = 0;
+	buf->head = 0;
+	return buf;
+}
+
+/*
+ * free pmu-private AUX data structures
+ */
+static void htm_free_aux(void *aux)
+{
+	struct htm_pmu_buf *buf = aux;
+
+	if (!buf)
+		return;
+
+	kfree(buf);
 }
 
 static struct pmu htm_pmu = {
@@ -386,7 +597,10 @@ static struct pmu htm_pmu = {
 	.read         = htm_event_read,
 	.start        = htm_event_start,
 	.stop         = htm_event_stop,
-	.capabilities = PERF_PMU_CAP_NO_EXCLUDE | PERF_PMU_CAP_EXCLUSIVE,
+	.setup_aux    = htm_setup_aux,
+	.free_aux     = htm_free_aux,
+	.capabilities = PERF_PMU_CAP_NO_EXCLUDE | PERF_PMU_CAP_EXCLUSIVE
+			| PERF_PMU_CAP_AUX_NO_SG | PERF_PMU_CAP_AUX_PREFER_LARGE,
 };
 
 static int htm_init(void)
-- 
2.43.0