[PATCH v3 3/6] firmware: Add RNMI handler infrastructure
Evgeny Voevodin
evvoevod at tenstorrent.com
Thu May 7 11:08:04 PDT 2026
Implement basic Resumable NMI (RNMI) handler support for the RISC-V
Smrnmi extension.
The new _trap_rnmi_handler assembly entry point saves context using the
Smrnmi MN* CSRs (MNSCRATCH, MNEPC, MNSTATUS, MNCAUSE) and returns via
mnret. It dispatches to sbi_trap_rnmi_handler(), which optionally calls
a platform-specific ops->rnmi_handler callback for actual NMI
processing. If no platform handler is registered or it fails, the
event is reported as an unhandled NMI.
The RNMI handler reuses the generic trap context structure but stores MN*
CSR values (MNEPC, MNSTATUS, MNCAUSE) into the corresponding generic
fields (mepc, mstatus, cause) for compatibility with existing trap
infrastructure.
Signed-off-by: Evgeny Voevodin <evvoevod at tenstorrent.com>
Reviewed-by: Anup Patel <anup at brainfault.org>
---
firmware/fw_base.S | 121 +++++++++++++++++++++++++++++++++++++
include/sbi/sbi_platform.h | 4 ++
include/sbi/sbi_trap.h | 2 +
lib/sbi/sbi_trap.c | 39 ++++++++++++
4 files changed, 166 insertions(+)
diff --git a/firmware/fw_base.S b/firmware/fw_base.S
index cdb15429..043de7d7 100644
--- a/firmware/fw_base.S
+++ b/firmware/fw_base.S
@@ -529,6 +529,45 @@ memcmp:
csrrw tp, CSR_MSCRATCH, tp
.endm
+.macro TRAP_SAVE_AND_SETUP_SP_T0_NMI
+ /* Swap TP and MNSCRATCH (for RNMI) */
+ csrrw tp, CSR_MNSCRATCH, tp
+
+ /* Save T0 in scratch space */
+ REG_S t0, SBI_SCRATCH_TMP1_OFFSET(tp)
+
+ /*
+ * Set T0 to appropriate exception stack
+ *
+ * Came_From_M_Mode = ((MNSTATUS.MNPP < PRV_M) ? 1 : 0) - 1;
+ * Exception_Stack = TP ^ (Came_From_M_Mode & (SP ^ TP))
+ */
+ csrr t0, CSR_MNSTATUS
+ srl t0, t0, 11 /* MNPP is at bits 11-12 */
+ and t0, t0, PRV_M
+ slti t0, t0, PRV_M
+ add t0, t0, -1
+ xor sp, sp, tp
+ and t0, t0, sp
+ xor sp, sp, tp
+ xor t0, tp, t0
+
+ /* Save original SP on exception stack */
+ REG_S sp, (SBI_TRAP_REGS_OFFSET(sp) - SBI_TRAP_CONTEXT_SIZE)(t0)
+
+ /* Set SP to exception stack and make room for trap context */
+ add sp, t0, -(SBI_TRAP_CONTEXT_SIZE)
+
+ /* Restore T0 from scratch space */
+ REG_L t0, SBI_SCRATCH_TMP1_OFFSET(tp)
+
+ /* Save T0 on stack */
+ REG_S t0, SBI_TRAP_REGS_OFFSET(t0)(sp)
+
+ /* Swap TP and MNSCRATCH */
+ csrrw tp, CSR_MNSCRATCH, tp
+.endm
+
.macro TRAP_SAVE_MEPC_MSTATUS have_mstatush
/* Save MEPC and MSTATUS CSRs */
csrr t0, CSR_MEPC
@@ -543,6 +582,20 @@ memcmp:
.endif
.endm
+.macro TRAP_SAVE_MNEPC_MNSTATUS have_mstatush
+ /*
+ * Save MNEPC and MNSTATUS CSRs (for RNMI)
+ * Note: Trap context structure has generic field names (mepc, mstatus),
+ * we store MN* CSR values into these same structure fields.
+ */
+ csrr t0, CSR_MNEPC
+ REG_S t0, SBI_TRAP_REGS_OFFSET(mepc)(sp)
+ csrr t0, CSR_MNSTATUS
+ REG_S t0, SBI_TRAP_REGS_OFFSET(mstatus)(sp)
+ /* MNSTATUSH doesn't exist in SMRNMI spec */
+ REG_S zero, SBI_TRAP_REGS_OFFSET(mstatusH)(sp)
+.endm
+
.macro TRAP_SAVE_GENERAL_REGS_EXCEPT_SP_T0
/* Save all general regisers except SP and T0 */
REG_S zero, SBI_TRAP_REGS_OFFSET(zero)(sp)
@@ -606,12 +659,36 @@ memcmp:
CLEAR_MDT t0
.endm
+.macro TRAP_SAVE_NMI_INFO
+ /*
+ * Save NMI trap info (MNCAUSE, no MNTVAL in spec)
+ * Note: Trap info structure has generic field names (cause, tval, etc.),
+ * we store MN* CSR values into these same structure fields.
+ */
+ csrr t0, CSR_MNCAUSE
+ REG_S t0, (SBI_TRAP_REGS_SIZE + SBI_TRAP_INFO_OFFSET(cause))(sp)
+ /* MNTVAL doesn't exist in SMRNMI spec */
+ REG_S zero, (SBI_TRAP_REGS_SIZE + SBI_TRAP_INFO_OFFSET(tval))(sp)
+ REG_S zero, (SBI_TRAP_REGS_SIZE + SBI_TRAP_INFO_OFFSET(tval2))(sp)
+ REG_S zero, (SBI_TRAP_REGS_SIZE + SBI_TRAP_INFO_OFFSET(tinst))(sp)
+ REG_S zero, (SBI_TRAP_REGS_SIZE + SBI_TRAP_INFO_OFFSET(gva))(sp)
+
+ /* We are ready to take another trap, clear MDT */
+ CLEAR_MDT t0
+.endm
+
.macro TRAP_CALL_C_ROUTINE
/* Call C routine */
add a0, sp, zero
call sbi_trap_handler
.endm
+.macro TRAP_CALL_C_RNMI_ROUTINE
+ /* Call C routine */
+ add a0, sp, zero
+ call sbi_trap_rnmi_handler
+.endm
+
.macro TRAP_RESTORE_GENERAL_REGS_EXCEPT_A0_T0
/* Restore all general regisers except A0 and T0 */
REG_L ra, SBI_TRAP_REGS_OFFSET(ra)(a0)
@@ -660,6 +737,19 @@ memcmp:
csrw CSR_MEPC, t0
.endm
+.macro TRAP_RESTORE_MNEPC_MNSTATUS
+ /*
+ * Restore MNSTATUS and MNEPC CSRs (for RNMI)
+ * Note: Load from generic structure fields (mstatus, mepc) and
+ * restore to NMI-specific CSRs (MNSTATUS, MNEPC).
+ * No MNSTATUSH in SMRNMI spec.
+ */
+ REG_L t0, SBI_TRAP_REGS_OFFSET(mstatus)(a0)
+ csrw CSR_MNSTATUS, t0
+ REG_L t0, SBI_TRAP_REGS_OFFSET(mepc)(a0)
+ csrw CSR_MNEPC, t0
+.endm
+
.macro TRAP_RESTORE_A0_T0
/* Restore T0 */
REG_L t0, SBI_TRAP_REGS_OFFSET(t0)(a0)
@@ -724,6 +814,37 @@ _trap_handler_hyp:
mret
+ .section .entry, "ax", %progbits
+ .align 3
+ .globl _trap_rnmi_handler
+_trap_rnmi_handler:
+ /*
+ * NMI interrupt handler using MN* CSRs
+ *
+ * Context detection via MNPP (previous privilege mode):
+ * - If MNPP < M-mode: use exception stack (TP)
+ * - If MNPP == M-mode: use current stack (SP)
+ * This handles nested interrupt cases.
+ */
+ TRAP_SAVE_AND_SETUP_SP_T0_NMI
+
+ TRAP_SAVE_MNEPC_MNSTATUS 0
+
+ TRAP_SAVE_GENERAL_REGS_EXCEPT_SP_T0
+
+ TRAP_SAVE_NMI_INFO
+
+ TRAP_CALL_C_RNMI_ROUTINE
+
+ TRAP_RESTORE_GENERAL_REGS_EXCEPT_A0_T0
+
+ TRAP_RESTORE_MNEPC_MNSTATUS
+
+ TRAP_RESTORE_A0_T0
+
+ /* mnret - return from NMI (SMRNMI extension) */
+ .word 0x70200073
+
.section .entry, "ax", %progbits
.align 3
.globl _reset_regs
diff --git a/include/sbi/sbi_platform.h b/include/sbi/sbi_platform.h
index e65d9877..715df499 100644
--- a/include/sbi/sbi_platform.h
+++ b/include/sbi/sbi_platform.h
@@ -149,6 +149,10 @@ struct sbi_platform_operations {
unsigned long log2len);
/** platform specific pmp disable on current HART */
void (*pmp_disable)(unsigned int n);
+
+ /** 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);
};
/** Platform default per-HART stack size for exception/interrupt handling */
diff --git a/include/sbi/sbi_trap.h b/include/sbi/sbi_trap.h
index 731a0c98..091a2446 100644
--- a/include/sbi/sbi_trap.h
+++ b/include/sbi/sbi_trap.h
@@ -289,6 +289,8 @@ static inline void sbi_trap_set_context(struct sbi_scratch *scratch,
struct sbi_trap_context *sbi_trap_handler(struct sbi_trap_context *tcntx);
+struct sbi_trap_context *sbi_trap_rnmi_handler(struct sbi_trap_context *tcntx);
+
#endif
#endif
diff --git a/lib/sbi/sbi_trap.c b/lib/sbi/sbi_trap.c
index f41db4d1..1e55b885 100644
--- a/lib/sbi/sbi_trap.c
+++ b/lib/sbi/sbi_trap.c
@@ -20,6 +20,7 @@
#include <sbi/sbi_irqchip.h>
#include <sbi/sbi_trap_ldst.h>
#include <sbi/sbi_pmu.h>
+#include <sbi/sbi_platform.h>
#include <sbi/sbi_scratch.h>
#include <sbi/sbi_sse.h>
#include <sbi/sbi_timer.h>
@@ -375,3 +376,41 @@ trap_done:
sbi_trap_set_context(scratch, tcntx->prev_context);
return tcntx;
}
+
+/**
+ * Default Resumable NMI (RNMI) handler
+ *
+ * This function is called from the _trap_rnmi_handler assembly code.
+ * It provides a simple wrapper that calls the platform-specific
+ * NMI handler if registered. If no handler is registered, it prints
+ * diagnostic information and hangs, similar to unhandled traps.
+ *
+ * Note: The trap context stores NMI CSR values (MNCAUSE, MNEPC, MNSTATUS)
+ * in the generic trap context fields (cause, mepc, mstatus).
+ *
+ * @param tcntx Pointer to trap context (saved on stack)
+ * @return Same trap context pointer (needed for restore macros)
+ */
+struct sbi_trap_context *sbi_trap_rnmi_handler(struct sbi_trap_context *tcntx)
+{
+ int rc;
+ const struct sbi_platform *plat = sbi_platform_thishart_ptr();
+ const struct sbi_platform_operations *ops = sbi_platform_ops(plat);
+
+ /* Call platform-specific NMI handler if registered */
+ if (ops && ops->rnmi_handler) {
+ rc = ops->rnmi_handler(tcntx);
+ if (rc) {
+ /* Platform handler failed to handle NMI */
+ sbi_trap_error("platform NMI handler failed", rc, tcntx);
+ }
+ return tcntx;
+ }
+
+ /* No platform handler - treat as unhandled NMI */
+ sbi_trap_error("unhandled NMI (no platform rnmi_handler)",
+ SBI_ENOTSUPP, tcntx);
+
+ /* Never returns */
+ return tcntx;
+}
--
2.43.0
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