RISC-V: Add hwprobe vDSO function and data

Add a vDSO function __vdso_riscv_hwprobe, which can sit in front of the
riscv_hwprobe syscall and answer common queries. We stash a copy of
static answers for the "all CPUs" case in the vDSO data page. This data
is private to the vDSO, so we can decide later to change what's stored
there or under what conditions we defer to the syscall. Currently all
data can be discovered at boot, so the vDSO function answers all queries
when the cpumask is set to the "all CPUs" hint.

There's also a boolean in the data that lets the vDSO function know that
all CPUs are the same. In that case, the vDSO will also answer queries
for arbitrary CPU masks in addition to the "all CPUs" hint.

Signed-off-by: Evan Green <evan@rivosinc.com>
Link: https://lore.kernel.org/r/20230407231103.2622178-7-evan@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

1 files changed, 52 insertions, 0 deletions

diff --git a/arch/riscv/kernel/vdso/hwprobe.c b/arch/riscv/kernel/vdso/hwprobe.c
new file mode 100644
index 000000000000..d40bec6ac078
--- /dev/null
+++ b/arch/riscv/kernel/vdso/hwprobe.c
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright 2023 Rivos, Inc
+ */
+
+#include <linux/types.h>
+#include <vdso/datapage.h>
+#include <vdso/helpers.h>
+
+extern int riscv_hwprobe(struct riscv_hwprobe *pairs, size_t pair_count,
+			 size_t cpu_count, unsigned long *cpus,
+			 unsigned int flags);
+
+/* Add a prototype to avoid -Wmissing-prototypes warning. */
+int __vdso_riscv_hwprobe(struct riscv_hwprobe *pairs, size_t pair_count,
+			 size_t cpu_count, unsigned long *cpus,
+			 unsigned int flags);
+
+int __vdso_riscv_hwprobe(struct riscv_hwprobe *pairs, size_t pair_count,
+			 size_t cpu_count, unsigned long *cpus,
+			 unsigned int flags)
+{
+	const struct vdso_data *vd = __arch_get_vdso_data();
+	const struct arch_vdso_data *avd = &vd->arch_data;
+	bool all_cpus = !cpu_count && !cpus;
+	struct riscv_hwprobe *p = pairs;
+	struct riscv_hwprobe *end = pairs + pair_count;
+
+	/*
+	 * Defer to the syscall for exotic requests. The vdso has answers
+	 * stashed away only for the "all cpus" case. If all CPUs are
+	 * homogeneous, then this function can handle requests for arbitrary
+	 * masks.
+	 */
+	if ((flags != 0) || (!all_cpus && !avd->homogeneous_cpus))
+		return riscv_hwprobe(pairs, pair_count, cpu_count, cpus, flags);
+
+	/* This is something we can handle, fill out the pairs. */
+	while (p < end) {
+		if (p->key <= RISCV_HWPROBE_MAX_KEY) {
+			p->value = avd->all_cpu_hwprobe_values[p->key];
+
+		} else {
+			p->key = -1;
+			p->value = 0;
+		}
+
+		p++;
+	}
+
+	return 0;
+}