[PATCH] lib: sbi: Apply budget restriction when polling Zkr CSR state transition

[Evgeny Voevodin]

Zkr architecture doesn't define a time limit on state transitions
which results in hanging on unresponsive or event-driven platforms.
To prevent this, we need to limit polling iterations and fall back
in case the budget is over, and stack guard keeps its initial value.
The budget is configurable with CONFIG_SBI_INIT_ZKR_POLL_BUDGET,
defaulting to 1000 iterations.
Successful reads do not consume a try.

Signed-off-by: Evgeny Voevodin <evvoevod at tenstorrent.com>
---
 lib/sbi/Kconfig    | 12 ++++++++++++
 lib/sbi/sbi_init.c | 12 +++++++++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/lib/sbi/Kconfig b/lib/sbi/Kconfig
index c6cc04bc..a11f788c 100644
--- a/lib/sbi/Kconfig
+++ b/lib/sbi/Kconfig
@@ -6,6 +6,18 @@ config CONSOLE_EARLY_BUFFER_SIZE
 	int "Early console buffer size (bytes)"
 	default 256
 
+config SBI_INIT_ZKR_POLL_BUDGET
+	int "Zkr seed polling budget (iterations)"
+	default 1000
+	help
+	  Maximum number of iterations to poll CSR_SEED when initializing
+	  the stack guard variable. The Zkr specification doesn't define
+	  a time limit on transitioning to ES16 between polls, which
+	  makes it impossible to tell whether entropy is being
+	  accumulated slowly or the entropy source is not functioning.
+	  This also limits the wait time on systems with an event-driven
+	  entropy source. A successful read doesn't consume a try.
+
 config SBI_ECALL_TIME
 	bool "Timer extension"
 	default y
diff --git a/lib/sbi/sbi_init.c b/lib/sbi/sbi_init.c
index b248e73f..7a0c4f74 100644
--- a/lib/sbi/sbi_init.c
+++ b/lib/sbi/sbi_init.c
@@ -280,20 +280,26 @@ static void __noreturn init_coldboot(struct sbi_scratch *scratch, u32 hartid)
 	if (sbi_hart_has_extension(scratch, SBI_HART_EXT_ZKR)) {
 		unsigned long guard_val = 0;
 		int chunks = sizeof(unsigned long) / sizeof(uint16_t);
-		bool res = true;
+		unsigned int tries = CONFIG_SBI_INIT_ZKR_POLL_BUDGET;
+		bool res = false;
 
-		while (chunks) {
+		while (chunks && tries) {
 			unsigned long seed = csr_swap(CSR_SEED, 0);
 			unsigned long opst = seed & SEED_OPTS_MASK;
+			res = false;
 
 			if (opst == SEED_OPTS_DEAD) {
-				res = false;
 				break;
 			}
 			if (opst == SEED_OPTS_ES16) {
 				guard_val = (guard_val << 16) | (seed & SEED_ENTROPY_MASK);
 				chunks--;
+				res = true;
+				/* Successful read doesn't consume a try */
+				tries++;
 			}
+
+			tries--;
 			continue;
 		}
 		if (res)
-- 
2.43.0