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, 248 insertions, 0 deletions

diff --git a/arch/powerpc/boot/addnote.c b/arch/powerpc/boot/addnote.c
new file mode 100644
index 000000000..787049274
--- /dev/null
+++ b/arch/powerpc/boot/addnote.c
@@ -0,0 +1,248 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Program to hack in a PT_NOTE program header entry in an ELF file.
+ * This is needed for OF on RS/6000s to load an image correctly.
+ * Note that OF needs a program header entry for the note, not an
+ * ELF section.
+ *
+ * Copyright 2000 Paul Mackerras.
+ *
+ * Adapted for 64 bit little endian images by Andrew Tauferner.
+ *
+ * Usage: addnote zImage
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <string.h>
+
+/* CHRP note section */
+static const char arch[] = "PowerPC";
+
+#define N_DESCR	6
+unsigned int descr[N_DESCR] = {
+	0xffffffff,		/* real-mode = true */
+	0x02000000,		/* real-base, i.e. where we expect OF to be */
+	0xffffffff,		/* real-size */
+	0xffffffff,		/* virt-base */
+	0xffffffff,		/* virt-size */
+	0x4000,			/* load-base */
+};
+
+/* RPA note section */
+static const char rpaname[] = "IBM,RPA-Client-Config";
+
+/*
+ * Note: setting ignore_my_client_config *should* mean that OF ignores
+ * all the other fields, but there is a firmware bug which means that
+ * it looks at the splpar field at least.  So these values need to be
+ * reasonable.
+ */
+#define N_RPA_DESCR	8
+unsigned int rpanote[N_RPA_DESCR] = {
+	0,			/* lparaffinity */
+	64,			/* min_rmo_size */
+	0,			/* min_rmo_percent */
+	40,			/* max_pft_size */
+	1,			/* splpar */
+	-1,			/* min_load */
+	0,			/* new_mem_def */
+	1,			/* ignore_my_client_config */
+};
+
+#define ROUNDUP(len)	(((len) + 3) & ~3)
+
+unsigned char buf[1024];
+#define ELFDATA2LSB     1
+#define ELFDATA2MSB     2
+static int e_data = ELFDATA2MSB;
+#define ELFCLASS32      1
+#define ELFCLASS64      2
+static int e_class = ELFCLASS32;
+
+#define GET_16BE(off)	((buf[off] << 8) + (buf[(off)+1]))
+#define GET_32BE(off)	((GET_16BE(off) << 16U) + GET_16BE((off)+2U))
+#define GET_64BE(off)	((((unsigned long long)GET_32BE(off)) << 32ULL) + \
+			((unsigned long long)GET_32BE((off)+4ULL)))
+#define PUT_16BE(off, v)(buf[off] = ((v) >> 8) & 0xff, \
+			 buf[(off) + 1] = (v) & 0xff)
+#define PUT_32BE(off, v)(PUT_16BE((off), (v) >> 16L), PUT_16BE((off) + 2, (v)))
+#define PUT_64BE(off, v)((PUT_32BE((off), (unsigned long long)(v) >> 32L), \
+			  PUT_32BE((off) + 4, (unsigned long long)(v))))
+
+#define GET_16LE(off)	((buf[off]) + (buf[(off)+1] << 8))
+#define GET_32LE(off)	(GET_16LE(off) + (GET_16LE((off)+2U) << 16U))
+#define GET_64LE(off)	((unsigned long long)GET_32LE(off) + \
+			(((unsigned long long)GET_32LE((off)+4ULL)) << 32ULL))
+#define PUT_16LE(off, v) (buf[off] = (v) & 0xff, \
+			  buf[(off) + 1] = ((v) >> 8) & 0xff)
+#define PUT_32LE(off, v) (PUT_16LE((off), (v)), PUT_16LE((off) + 2, (v) >> 16L))
+#define PUT_64LE(off, v) (PUT_32LE((off), (unsigned long long)(v)), \
+			  PUT_32LE((off) + 4, (unsigned long long)(v) >> 32L))
+
+#define GET_16(off)	(e_data == ELFDATA2MSB ? GET_16BE(off) : GET_16LE(off))
+#define GET_32(off)	(e_data == ELFDATA2MSB ? GET_32BE(off) : GET_32LE(off))
+#define GET_64(off)	(e_data == ELFDATA2MSB ? GET_64BE(off) : GET_64LE(off))
+#define PUT_16(off, v)	(e_data == ELFDATA2MSB ? PUT_16BE(off, v) : \
+			 PUT_16LE(off, v))
+#define PUT_32(off, v)  (e_data == ELFDATA2MSB ? PUT_32BE(off, v) : \
+			 PUT_32LE(off, v))
+#define PUT_64(off, v)  (e_data == ELFDATA2MSB ? PUT_64BE(off, v) : \
+			 PUT_64LE(off, v))
+
+/* Structure of an ELF file */
+#define E_IDENT		0	/* ELF header */
+#define	E_PHOFF		(e_class == ELFCLASS32 ? 28 : 32)
+#define E_PHENTSIZE	(e_class == ELFCLASS32 ? 42 : 54)
+#define E_PHNUM		(e_class == ELFCLASS32 ? 44 : 56)
+#define E_HSIZE		(e_class == ELFCLASS32 ? 52 : 64)
+
+#define EI_MAGIC	0	/* offsets in E_IDENT area */
+#define EI_CLASS	4
+#define EI_DATA		5
+
+#define PH_TYPE		0	/* ELF program header */
+#define PH_OFFSET	(e_class == ELFCLASS32 ? 4 : 8)
+#define PH_FILESZ	(e_class == ELFCLASS32 ? 16 : 32)
+#define PH_HSIZE	(e_class == ELFCLASS32 ? 32 : 56)
+
+#define PT_NOTE		4	/* Program header type = note */
+
+
+unsigned char elf_magic[4] = { 0x7f, 'E', 'L', 'F' };
+
+int
+main(int ac, char **av)
+{
+	int fd, n, i;
+	unsigned long ph, ps, np;
+	long nnote, nnote2, ns;
+
+	if (ac != 2) {
+		fprintf(stderr, "Usage: %s elf-file\n", av[0]);
+		exit(1);
+	}
+	fd = open(av[1], O_RDWR);
+	if (fd < 0) {
+		perror(av[1]);
+		exit(1);
+	}
+
+	nnote = 12 + ROUNDUP(strlen(arch) + 1) + sizeof(descr);
+	nnote2 = 12 + ROUNDUP(strlen(rpaname) + 1) + sizeof(rpanote);
+
+	n = read(fd, buf, sizeof(buf));
+	if (n < 0) {
+		perror("read");
+		exit(1);
+	}
+
+	if (memcmp(&buf[E_IDENT+EI_MAGIC], elf_magic, 4) != 0)
+		goto notelf;
+	e_class = buf[E_IDENT+EI_CLASS];
+	if (e_class != ELFCLASS32 && e_class != ELFCLASS64)
+		goto notelf;
+	e_data = buf[E_IDENT+EI_DATA];
+	if (e_data != ELFDATA2MSB && e_data != ELFDATA2LSB)
+		goto notelf;
+	if (n < E_HSIZE)
+		goto notelf;
+
+	ph = (e_class == ELFCLASS32 ? GET_32(E_PHOFF) : GET_64(E_PHOFF));
+	ps = GET_16(E_PHENTSIZE);
+	np = GET_16(E_PHNUM);
+	if (ph < E_HSIZE || ps < PH_HSIZE || np < 1)
+		goto notelf;
+	if (ph + (np + 2) * ps + nnote + nnote2 > n)
+		goto nospace;
+
+	for (i = 0; i < np; ++i) {
+		if (GET_32(ph + PH_TYPE) == PT_NOTE) {
+			fprintf(stderr, "%s already has a note entry\n",
+				av[1]);
+			exit(0);
+		}
+		ph += ps;
+	}
+
+	/* XXX check that the area we want to use is all zeroes */
+	for (i = 0; i < 2 * ps + nnote + nnote2; ++i)
+		if (buf[ph + i] != 0)
+			goto nospace;
+
+	/* fill in the program header entry */
+	ns = ph + 2 * ps;
+	PUT_32(ph + PH_TYPE, PT_NOTE);
+	if (e_class == ELFCLASS32)
+		PUT_32(ph + PH_OFFSET, ns);
+	else
+		PUT_64(ph + PH_OFFSET, ns);
+
+	if (e_class == ELFCLASS32)
+		PUT_32(ph + PH_FILESZ, nnote);
+	else
+		PUT_64(ph + PH_FILESZ, nnote);
+
+	/* fill in the note area we point to */
+	/* XXX we should probably make this a proper section */
+	PUT_32(ns, strlen(arch) + 1);
+	PUT_32(ns + 4, N_DESCR * 4);
+	PUT_32(ns + 8, 0x1275);
+	strcpy((char *) &buf[ns + 12], arch);
+	ns += 12 + strlen(arch) + 1;
+	for (i = 0; i < N_DESCR; ++i, ns += 4)
+		PUT_32BE(ns, descr[i]);
+
+	/* fill in the second program header entry and the RPA note area */
+	ph += ps;
+	PUT_32(ph + PH_TYPE, PT_NOTE);
+	if (e_class == ELFCLASS32)
+		PUT_32(ph + PH_OFFSET, ns);
+	else
+		PUT_64(ph + PH_OFFSET, ns);
+
+	if (e_class == ELFCLASS32)
+		PUT_32(ph + PH_FILESZ, nnote);
+	else
+		PUT_64(ph + PH_FILESZ, nnote2);
+
+	/* fill in the note area we point to */
+	PUT_32(ns, strlen(rpaname) + 1);
+	PUT_32(ns + 4, sizeof(rpanote));
+	PUT_32(ns + 8, 0x12759999);
+	strcpy((char *) &buf[ns + 12], rpaname);
+	ns += 12 + ROUNDUP(strlen(rpaname) + 1);
+	for (i = 0; i < N_RPA_DESCR; ++i, ns += 4)
+		PUT_32BE(ns, rpanote[i]);
+
+	/* Update the number of program headers */
+	PUT_16(E_PHNUM, np + 2);
+
+	/* write back */
+	i = lseek(fd, (long) 0, SEEK_SET);
+	if (i < 0) {
+		perror("lseek");
+		exit(1);
+	}
+	i = write(fd, buf, n);
+	if (i < 0) {
+		perror("write");
+		exit(1);
+	}
+	if (i < n) {
+		fprintf(stderr, "%s: write truncated\n", av[1]);
+		exit(1);
+	}
+
+	exit(0);
+
+ notelf:
+	fprintf(stderr, "%s does not appear to be an ELF file\n", av[1]);
+	exit(1);
+
+ nospace:
+	fprintf(stderr, "sorry, I can't find space in %s to put the note\n",
+		av[1]);
+	exit(1);
+}