Fix RISC-V Linux build (#212)

Cpuinfo was failing to build on RISC-V Linux distributions, e.g.,
Ubuntu 23.10, as it includes a header file sys/hwprobe.h that is
not yet provided by glibc (although it is provided by bionic).  We
fix the issue by only including sys/hwprobe.h when building for
Android, and invoking the hwprobe syscall directly on other
Linux distributions.  The Android specific check can be removed in
the future once sys/hwprobe.h becomes available in glibc.

src/riscv/linux/riscv-hw.c

@@ -1,10 +1,74 @@
-#include <sys/hwprobe.h>
+/*
+ * Only enable the C standard library hwprobe interface on Android for now.
+ * Patches to add a compatible hwprobe API to glibc are available but not
+ * merged at the time of writing and so cannot easily be tested.  The
+ * #ifdef __ANDROID__ check will be removed in the future.
+ */
+#ifdef __ANDROID__
+	#ifdef __has_include
+		#if __has_include (<sys/hwprobe.h>)
+			#define CPUINFO_RISCV_LINUX_HAVE_C_HWPROBE
+			#include <sys/hwprobe.h>
+		#endif
+	#endif
+#endif
+
 #include <sched.h>
 
 #include <cpuinfo/log.h>
 #include <riscv/api.h>
 #include <riscv/linux/api.h>
 
+#ifndef CPUINFO_RISCV_LINUX_HAVE_C_HWPROBE
+
+	#include <unistd.h>
+	#include <sys/syscall.h>
+	#include <stdint.h>
+
+	struct riscv_hwprobe {
+		int64_t key;
+		uint64_t value;
+	};
+
+	/*
+	 * The standard C library our binary was compiled with does not support
+	 * hwprobe but the kernel on which we are running might do.  The
+	 * constants below are copied from
+	 * /usr/include/riscv64-linux-gnu/asm/hwprobe.h.  They allow us to
+	 * invoke the hwprobe syscall directly.  We duplicate the constants
+	 * rather than including the kernel hwprobe.h header, as this header
+	 * will only be present if we're building Linux 6.4 or greater.
+	 */
+
+	#define RISCV_HWPROBE_KEY_MVENDORID	0
+	#define RISCV_HWPROBE_KEY_MARCHID	1
+	#define RISCV_HWPROBE_KEY_MIMPID	2
+	#define RISCV_HWPROBE_KEY_BASE_BEHAVIOR	3
+	#define		RISCV_HWPROBE_BASE_BEHAVIOR_IMA	(1 << 0)
+	#define RISCV_HWPROBE_KEY_IMA_EXT_0	4
+	#define		RISCV_HWPROBE_IMA_FD		(1 << 0)
+	#define		RISCV_HWPROBE_IMA_C		(1 << 1)
+	#define		RISCV_HWPROBE_IMA_V		(1 << 2)
+	#define		RISCV_HWPROBE_EXT_ZBA		(1 << 3)
+	#define		RISCV_HWPROBE_EXT_ZBB		(1 << 4)
+	#define		RISCV_HWPROBE_EXT_ZBS		(1 << 5)
+	#define		RISCV_HWPROBE_EXT_ZICBOZ	(1 << 6)
+	#define RISCV_HWPROBE_KEY_CPUPERF_0	5
+	#define		RISCV_HWPROBE_MISALIGNED_UNKNOWN	(0 << 0)
+	#define		RISCV_HWPROBE_MISALIGNED_EMULATED	(1 << 0)
+	#define		RISCV_HWPROBE_MISALIGNED_SLOW		(2 << 0)
+	#define		RISCV_HWPROBE_MISALIGNED_FAST		(3 << 0)
+	#define		RISCV_HWPROBE_MISALIGNED_UNSUPPORTED	(4 << 0)
+	#define		RISCV_HWPROBE_MISALIGNED_MASK		(7 << 0)
+
+	#ifndef NR_riscv_hwprobe
+		#ifndef NR_arch_specific_syscall
+			#define NR_arch_specific_syscall 244
+		#endif
+		#define NR_riscv_hwprobe (NR_arch_specific_syscall + 14)
+	#endif
+#endif
+
 void cpuinfo_riscv_linux_decode_vendor_uarch_from_hwprobe(
 		uint32_t processor,
 		enum cpuinfo_vendor vendor[restrict static 1],
@@ -33,9 +97,23 @@ void cpuinfo_riscv_linux_decode_vendor_uarch_from_hwprobe(
 	CPU_SET_S(processor, cpu_set_size, cpu_set);
 
 	/* Request all available information from hwprobe. */
+#ifndef CPUINFO_RISCV_LINUX_HAVE_C_HWPROBE
+	/*
+	 * No standard library support for hwprobe.  We'll need to invoke the
+	 * syscall directly.  See
+	 *
+	 * https://docs.kernel.org/arch/riscv/hwprobe.html
+	 *
+	 * for more details.
+	 */
+	int ret = syscall(NR_riscv_hwprobe, pairs, pairs_count,
+			  cpu_set_size, (unsigned long*)cpu_set,
+			  0 /* flags */);
+#else
 	int ret = __riscv_hwprobe(pairs, pairs_count,
                                   cpu_set_size, (unsigned long*)cpu_set,
                                   0 /* flags */);
+#endif
 	if (ret < 0) {
 		cpuinfo_log_warning("failed to get hwprobe information, err: %d", ret);
 		goto cleanup;