[PATCH 3/4] lib: sbi: Add floating-point context save/restore support.

[dave.patel@riscstar.com]

From: Dave Patel <dave.patel@riscstar.com>

Add support for saving and restoring RISC-V floating-point (F/D) extension
state in OpenSBI. This introduces a per-hart floating-point context
structure and helper routines to perform full context save and restore.

The floating-point context includes storage for all 32 FPi registers (f0–f31)
along with the fcsr control and status register. The register state is saved
and restored using double-precision load/store instructions (fsd/fld),
assuming an RV64 system with D-extension support.

The implementation follows an eager context switching model where the entire
FP state is saved and restored on every context switch. This avoids the need
for trap-based lazy management and keeps the design simple and deterministic.

A per-hart pointer is used to track the current floating-point context owner.

Notes:
- The implementation assumes the floating-point unit is enabled
(mstatus.FS != Off) when invoked.
- The context layout is fixed to 64-bit FP registers.

Signed-off-by: Dave Patel <dave.patel@riscstar.com>"
---
 include/sbi/sbi_fp.h |  22 ++++++++
 lib/sbi/objects.mk   |   1 +
 lib/sbi/sbi_fp.c     | 117 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 140 insertions(+)
 create mode 100644 include/sbi/sbi_fp.h
 create mode 100644 lib/sbi/sbi_fp.c

diff --git a/include/sbi/sbi_fp.h b/include/sbi/sbi_fp.h
new file mode 100644
index 00000000..58c5b50e
--- /dev/null
+++ b/include/sbi/sbi_fp.h
@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright (c) 2026 RISCstar Solutions.
+ *
+ * Authors:
+ *     Dave Patel <dave.patel@riscstar.com>
+ */
+
+#ifndef __SBI_FP_H__
+#define __SBI_FP_H__
+
+struct sbi_fp_context {
+    unsigned long f[32];
+    unsigned long fcsr;
+};
+
+struct sbi_fp_context *sbi_current_fp_context(void);
+void sbi_fp_save(struct sbi_fp_context *dst);
+void sbi_fp_restore(const struct sbi_fp_context *src);
+
+#endif //__SBI_VECTOR_H__
+
diff --git a/lib/sbi/objects.mk b/lib/sbi/objects.mk
index 5c0caf39..ca560c2e 100644
--- a/lib/sbi/objects.mk
+++ b/lib/sbi/objects.mk
@@ -107,3 +107,4 @@ libsbi-objs-y += sbi_unpriv.o
 libsbi-objs-y += sbi_expected_trap.o
 libsbi-objs-y += sbi_cppc.o
 libsbi-objs-y += sbi_vector.o
+libsbi-objs-y += sbi_fp.o
diff --git a/lib/sbi/sbi_fp.c b/lib/sbi/sbi_fp.c
new file mode 100644
index 00000000..4d5e0f68
--- /dev/null
+++ b/lib/sbi/sbi_fp.c
@@ -0,0 +1,117 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright (c) 2026 RISCstar Solutions.
+ *
+ * Authors:
+ *     Dave Patel <dave.patel@riscstar.com>
+ */
+
+#include <sbi/riscv_asm.h>
+#include <sbi/riscv_encoding.h>
+#include <sbi/sbi_scratch.h>
+#include <sbi/sbi_fp.h>
+
+/* Per-hart current owner pointer (SMP-safe) */
+static inline struct sbi_fp_context **fp_owner_ptr(void)
+{
+    struct sbi_scratch *scratch = sbi_scratch_thishart_ptr();
+    return &scratch->fp_ctx;
+}
+
+/* Get current FP context */
+struct sbi_fp_context *sbi_current_fp_context(void)
+{
+    return *fp_owner_ptr();
+}
+
+void sbi_fp_save(struct sbi_fp_context *dst)
+{
+	if (!dst)
+		return;
+
+	asm volatile(
+		"fsd f0,  0(%0)\n"
+		"fsd f1,  8(%0)\n"
+		"fsd f2, 16(%0)\n"
+		"fsd f3, 24(%0)\n"
+		"fsd f4, 32(%0)\n"
+		"fsd f5, 40(%0)\n"
+		"fsd f6, 48(%0)\n"
+		"fsd f7, 56(%0)\n"
+		"fsd f8, 64(%0)\n"
+		"fsd f9, 72(%0)\n"
+		"fsd f10, 80(%0)\n"
+		"fsd f11, 88(%0)\n"
+		"fsd f12, 96(%0)\n"
+		"fsd f13, 104(%0)\n"
+		"fsd f14, 112(%0)\n"
+		"fsd f15, 120(%0)\n"
+		"fsd f16, 128(%0)\n"
+		"fsd f17, 136(%0)\n"
+		"fsd f18, 144(%0)\n"
+		"fsd f19, 152(%0)\n"
+		"fsd f20, 160(%0)\n"
+		"fsd f21, 168(%0)\n"
+		"fsd f22, 176(%0)\n"
+		"fsd f23, 184(%0)\n"
+		"fsd f24, 192(%0)\n"
+		"fsd f25, 200(%0)\n"
+		"fsd f26, 208(%0)\n"
+		"fsd f27, 216(%0)\n"
+		"fsd f28, 224(%0)\n"
+		"fsd f29, 232(%0)\n"
+		"fsd f30, 240(%0)\n"
+		"fsd f31, 248(%0)\n"
+		:
+		: "r"(dst->f)
+		: "memory"
+	);
+
+	dst->fcsr = csr_read(CSR_FCSR);
+}
+
+void sbi_fp_restore(const struct sbi_fp_context *src)
+{
+    if (!src)
+        return;
+
+	asm volatile(
+		"fld f0,  0(%0)\n"
+		"fld f1,  8(%0)\n"
+		"fld f2, 16(%0)\n"
+		"fld f3, 24(%0)\n"
+		"fld f4, 32(%0)\n"
+		"fld f5, 40(%0)\n"
+		"fld f6, 48(%0)\n"
+		"fld f7, 56(%0)\n"
+		"fld f8, 64(%0)\n"
+		"fld f9, 72(%0)\n"
+		"fld f10, 80(%0)\n"
+		"fld f11, 88(%0)\n"
+		"fld f12, 96(%0)\n"
+		"fld f13, 104(%0)\n"
+		"fld f14, 112(%0)\n"
+		"fld f15, 120(%0)\n"
+		"fld f16, 128(%0)\n"
+		"fld f17, 136(%0)\n"
+		"fld f18, 144(%0)\n"
+		"fld f19, 152(%0)\n"
+		"fld f20, 160(%0)\n"
+		"fld f21, 168(%0)\n"
+		"fld f22, 176(%0)\n"
+		"fld f23, 184(%0)\n"
+		"fld f24, 192(%0)\n"
+		"fld f25, 200(%0)\n"
+		"fld f26, 208(%0)\n"
+		"fld f27, 216(%0)\n"
+		"fld f28, 224(%0)\n"
+		"fld f29, 232(%0)\n"
+		"fld f30, 240(%0)\n"
+		"fld f31, 248(%0)\n"
+		:
+		: "r"(src->f)
+		: "memory"
+	);
+
+	csr_write(CSR_FCSR, src->fcsr);
+}
-- 
2.43.0


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