[PATCH v2 5/5] lib: sbi: hart: Detect and enable Smrnmi before trap-based feature detection

[Evgeny Voevodin]

The location of the RNMI/E trap vectors in the Smrnmi extension is
implementation-defined, so platforms with vendor-specific NMI vector
mechanisms must install the firmware's NMI entry points themselves.

Add an smrnmi_handlers_init() callback to sbi_platform_operations that
receives the firmware entry points and lets platform code install them
at the hardware-specific vector locations. Two pointers are passed:

  - _trap_rnmi_handler: the dedicated RNMI entry point that saves
    context using the Smrnmi MN* CSRs and returns via mnret.
  - _trap_handler: the regular M-mode trap entry since RNME is taken
    as a regular M-mode trap with NMIE=0.

When Smrnmi is present, install the platform's NMI vectors via the new
callback, initialize MNSCRATCH with the per-hart scratch pointer, and
set MNSTATUS.NMIE.

Smrnmi-enabled platforms must register smrnmi_handlers_init; if the
extension is detected but no callback is registered, sbi_panic() is
called since enabling NMIs without handlers in place would route
subsequent traps into nowhere.

Signed-off-by: Evgeny Voevodin <evvoevod at tenstorrent.com>
---
 include/sbi/sbi_platform.h |  4 ++++
 lib/sbi/sbi_hart.c         | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/include/sbi/sbi_platform.h b/include/sbi/sbi_platform.h
index 715df499..fe382b56 100644
--- a/include/sbi/sbi_platform.h
+++ b/include/sbi/sbi_platform.h
@@ -150,6 +150,10 @@ struct sbi_platform_operations {
 	/** platform specific pmp disable on current HART */
 	void (*pmp_disable)(unsigned int n);
 
+	/** platform specific Smrnmi handlers init on current HART */
+	void (*smrnmi_handlers_init)(void (*rnmi_handler)(void),
+			void (*rnme_handler)(void));
+
 	/** platform specific Smrnmi NMI handler.
 	 *  Returns SBI_SUCCESS on success, error code if NMI cannot be handled. */
 	int (*rnmi_handler)(struct sbi_trap_context *tcntx);
diff --git a/lib/sbi/sbi_hart.c b/lib/sbi/sbi_hart.c
index 781161e5..92c602aa 100644
--- a/lib/sbi/sbi_hart.c
+++ b/lib/sbi/sbi_hart.c
@@ -532,6 +532,26 @@ static int hart_detect_features(struct sbi_scratch *scratch)
 	if (rc)
 		return rc;
 
+	if (sbi_hart_has_extension(scratch, SBI_HART_EXT_SMRNMI)) {
+		const struct sbi_platform *plat = sbi_platform_thishart_ptr();
+		const struct sbi_platform_operations *ops = sbi_platform_ops(plat);
+		extern void _trap_rnmi_handler(void);
+		extern void _trap_handler(void);
+
+		if (!ops || !ops->smrnmi_handlers_init)
+			sbi_panic("Smrnmi detected, but platform lacks smrnmi_handlers_init callback\n");
+
+		/* Reuse _trap_handler for the RNME slot since RNME is taken
+		 * as a regular M-mode trap with NMIE=0. */
+		ops->smrnmi_handlers_init(_trap_rnmi_handler, _trap_handler);
+
+		/* Initialize MNSCRATCH for the RNMI handler */
+		csr_write(CSR_MNSCRATCH, scratch);
+
+		/* Enable NMIs */
+		csr_set(CSR_MNSTATUS, MNSTATUS_NMIE);
+	}
+
 #define __check_hpm_csr(__csr, __mask) 					  \
 	oldval = csr_read_allowed(__csr, &trap);			  \
 	if (!trap.cause) {						  \
-- 
2.43.0