sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 93 insertions, 0 deletions

diff --git a/arch/powerpc/boot/of.c b/arch/powerpc/boot/of.c
new file mode 100644
index 000000000..2fbd4ae60
--- /dev/null
+++ b/arch/powerpc/boot/of.c
@@ -0,0 +1,93 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) Paul Mackerras 1997.
+ */
+#include <stdarg.h>
+#include <stddef.h>
+#include "types.h"
+#include "elf.h"
+#include "string.h"
+#include "stdio.h"
+#include "page.h"
+#include "ops.h"
+
+#include "of.h"
+
+/* Value picked to match that used by yaboot */
+#define PROG_START	0x01400000	/* only used on 64-bit systems */
+#define RAM_END		(512<<20)	/* Fixme: use OF */
+#define	ONE_MB		0x100000
+
+
+
+static unsigned long claim_base;
+
+void epapr_platform_init(unsigned long r3, unsigned long r4, unsigned long r5,
+			 unsigned long r6, unsigned long r7);
+
+static void *of_try_claim(unsigned long size)
+{
+	unsigned long addr = 0;
+
+	if (claim_base == 0)
+		claim_base = _ALIGN_UP((unsigned long)_end, ONE_MB);
+
+	for(; claim_base < RAM_END; claim_base += ONE_MB) {
+#ifdef DEBUG
+		printf("    trying: 0x%08lx\n\r", claim_base);
+#endif
+		addr = (unsigned long) of_claim(claim_base, size, 0);
+		if (addr != PROM_ERROR)
+			break;
+	}
+	if (addr == 0)
+		return NULL;
+	claim_base = PAGE_ALIGN(claim_base + size);
+	return (void *)addr;
+}
+
+static void of_image_hdr(const void *hdr)
+{
+	const Elf64_Ehdr *elf64 = hdr;
+
+	if (elf64->e_ident[EI_CLASS] == ELFCLASS64) {
+		/*
+		 * Maintain a "magic" minimum address. This keeps some older
+		 * firmware platforms running.
+		 */
+		if (claim_base < PROG_START)
+			claim_base = PROG_START;
+	}
+}
+
+static void of_platform_init(unsigned long a1, unsigned long a2, void *promptr)
+{
+	platform_ops.image_hdr = of_image_hdr;
+	platform_ops.malloc = of_try_claim;
+	platform_ops.exit = of_exit;
+	platform_ops.vmlinux_alloc = of_vmlinux_alloc;
+
+	dt_ops.finddevice = of_finddevice;
+	dt_ops.getprop = of_getprop;
+	dt_ops.setprop = of_setprop;
+
+	of_console_init();
+
+	of_init(promptr);
+	loader_info.promptr = promptr;
+	if (a1 && a2 && a2 != 0xdeadbeef) {
+		loader_info.initrd_addr = a1;
+		loader_info.initrd_size = a2;
+	}
+}
+
+void platform_init(unsigned long r3, unsigned long r4, unsigned long r5,
+		   unsigned long r6, unsigned long r7)
+{
+	/* Detect OF vs. ePAPR boot */
+	if (r5)
+		of_platform_init(r3, r4, (void *)r5);
+	else
+		epapr_platform_init(r3, r4, r5, r6, r7);
+}
+