supervisor: Add an example on the HSM extension

The HSM extension from SBI v2.0 allows a supervisor to manage the state
of harts on the system. Moreover, it also guarantees that only one hart
makes it into the booting stages of the supervisor, and it's up to the
supervisor to wake up the rest through the HSM extension.

I have added an example which showcases this extension in different
ways, and it finally shuts down the machine.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 215 insertions, 0 deletions

diff --git a/arch/riscv/supervisor/hsm/hsm.c b/arch/riscv/supervisor/hsm/hsm.c
new file mode 100644
index 0000000..0b2daca
--- /dev/null
+++ b/arch/riscv/supervisor/hsm/hsm.c
@@ -0,0 +1,215 @@
+#include "compiler.h"
+#include "config.h"
+#include "sbi.h"
+
+// Implemented in head.S. Here we need the symbol in order to pass the address
+// of the function when starting secondary harts.
+extern void _start(void);
+
+/*
+ * Similar to the Linux kernel and what I did in https://github.com/mssola/fbos,
+ * harts will enter in a lottery when starting. See the README.md file for more
+ * details on this.
+ */
+atomic_t hart_lottery __attribute__((__section__(".sdata")));
+atomic_t hart_failed_lottery __attribute__((__section__(".sdata")));
+
+// Fake a delay so it's easier on the eyes.
+#define SLEEP()                            \
+	for (int i = 0; i < 1000000000; i++) { \
+	}
+
+/*
+ * Printing utilities.
+ */
+
+void printk(const char *const message);
+
+void write(const char *const message, size_t n)
+{
+	struct sbi_ret ret = sbi_ecall2(DBCN_EXT, DBCN_WRITE, n, (unsigned long)message);
+	if (ret.error != SBI_SUCCESS) {
+		for (;;) {
+			;
+		}
+	}
+}
+
+size_t strlen(const char *const message)
+{
+	size_t i = 0;
+
+	for (; message[i] != '\0'; i++)
+		;
+
+	return i;
+}
+
+__attribute__((__noreturn__)) void die(const char *const message)
+{
+	if (message) {
+		printk(message);
+	}
+
+	for (;;)
+		;
+}
+
+void print_digit(uint64_t digit)
+{
+	char buffer[2];
+
+	if (digit > 9) {
+		die("We cannot print numbers with two or more digits :D\n");
+	}
+
+	buffer[0] = '0' + digit;
+	buffer[1] = '\0';
+	write(buffer, 2);
+}
+
+void printk(const char *const message)
+{
+	size_t len = strlen(message);
+	if (!len) {
+		return;
+	}
+
+	write(message, len);
+}
+
+/*
+ * Printing/asserting the stats of the lottery.
+ */
+
+void assert_lottery(uint64_t participated, uint64_t failed)
+{
+	if (hart_lottery.value != participated) {
+		die("Bad number for participating harts\n");
+	}
+	if (hart_failed_lottery.value != failed) {
+		die("Bad number for failed harts\n");
+	}
+}
+
+void print_lottery(void)
+{
+	printk("How many harts have _participated_ in the lottery? ");
+	print_digit(hart_lottery.value);
+	write("\n", 1);
+
+	printk("How many harts have _failed_ in the lottery? ");
+	print_digit(hart_failed_lottery.value);
+	write("\n", 1);
+}
+
+// Called at the very end of the program. It will shutdown the machine.
+void end(void)
+{
+	printk("\nTHE END\n");
+
+	// SBI call to the SRST extension to gracefully shutdown the machine.
+	sbi_ecall2(SRST_EXT, SRST_RESET, SRST_SHUTDOWN, SRST_NO_REASON);
+}
+
+void start_kernel(uint64_t hart_id, uintptr_t opaque)
+{
+	printk("Hello, world!\n");
+
+	/*
+	 * Figure out the harts that are meant to be stopped for now.
+	 */
+
+	unsigned int other_harts[NR_CPUS - 1];
+	struct sbi_ret ret;
+
+	printk("Hart that won the lottery: ");
+	print_digit(hart_id);
+	write("\n", 1);
+
+	printk("Hart still asleep: ");
+	for (unsigned int i = 0, j = 0; i < NR_CPUS; i++) {
+		if (i != hart_id) {
+			print_digit(i);
+			if (j + 1 < NR_CPUS - 1) {
+				write(", ", 2);
+			}
+			other_harts[j++] = i;
+		}
+	}
+	write("\n", 1);
+
+	/*
+	 * ASSERT: all harts in 'other_harts' are marked as stopped by HSM.
+	 */
+
+	for (unsigned int i = 0; i < NR_CPUS - 1; i++) {
+		ret = sbi_ecall1(HSM_EXT, HSM_HART_GET_STATUS, other_harts[i]);
+
+		if (ret.error != SBI_SUCCESS) {
+			die("HSM_HART_GET_STATUS: unexpected error from SBI\n");
+		}
+		if (ret.value != HSM_HART_STOPPED) {
+			die("HSM_HART_GET_STATUS: hart that was supposed to be stopped is not!\n");
+		}
+	}
+
+	// Initial state of the lottery.
+	print_lottery();
+	assert_lottery(1, 0);
+	write("\n", 1);
+
+	/*
+	 * Wake up the first hart.
+	 */
+
+	printk("Waking up first secondary hart\n");
+
+	ret = sbi_ecall3(HSM_EXT, HSM_HART_START, other_harts[0], (uintptr_t)&_start, opaque);
+	if (ret.error != SBI_SUCCESS) {
+		die("HSM_HART_START: unexpected error from SBI\n");
+	}
+
+	SLEEP();
+
+	ret = sbi_ecall1(HSM_EXT, HSM_HART_GET_STATUS, other_harts[0]);
+	if (ret.error != SBI_SUCCESS) {
+		die("HSM_HART_GET_STATUS: unexpected error from SBI\n");
+	}
+	if (ret.value != HSM_HART_STARTED) {
+		die("HSM_HART_GET_STATUS: hart that was supposed to be started is not!\n");
+	}
+
+	// Another hart should have entered the lottery and failed.
+	print_lottery();
+	assert_lottery(2, 1);
+	write("\n", 1);
+
+	/*
+	 * Now wake up the rest.
+	 */
+
+	printk("Waking up the rest\n");
+
+	for (unsigned int i = 1; i < NR_CPUS - 1; i++) {
+		ret = sbi_ecall3(HSM_EXT, HSM_HART_START, other_harts[i], (uintptr_t)&_start, opaque);
+		if (ret.error != SBI_SUCCESS) {
+			die("HSM_HART_START: unexpected error from SBI\n");
+		}
+	}
+
+	SLEEP();
+
+	// All awake, only one good.
+	print_lottery();
+	assert_lottery(NR_CPUS, NR_CPUS - 1);
+	write("\n", 1);
+
+	/*
+	 * Kernels usually don't quit, but let's do it for the heck of it; head.S
+	 * will handle it.
+	 */
+
+	printk("Goodbye, cruel world!\n");
+	SLEEP();
+}