[RFC V2 PATCH 6/9] riscv: add support for SBI misalignment trap delegation

["Clément Léger" <cleger@rivosinc.com>]

Add support for misalignment trap delegation by setting it with
SBI_EXT_FW_FEATURE SBI extension. This extension allows to control SBI
behavior by requesting to set them to specific value. In order to
implement prctl(PR_SET_UNALIGN, PR_UNALIGN_SIGBUS) behavior properly, we
need to let the kernel handle the misalignment error by itself.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
---
 arch/riscv/include/asm/sbi.h | 11 +++++++++++
 arch/riscv/kernel/sbi.c      | 21 +++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/arch/riscv/include/asm/sbi.h b/arch/riscv/include/asm/sbi.h
index 5b4a1bf5f439..c1b74c7d0d56 100644
--- a/arch/riscv/include/asm/sbi.h
+++ b/arch/riscv/include/asm/sbi.h
@@ -30,6 +30,7 @@ enum sbi_ext_id {
 	SBI_EXT_HSM = 0x48534D,
 	SBI_EXT_SRST = 0x53525354,
 	SBI_EXT_PMU = 0x504D55,
+	SBI_EXT_FW_FEATURE = 0x46574654,
 
 	/* Experimentals extensions must lie within this range */
 	SBI_EXT_EXPERIMENTAL_START = 0x08000000,
@@ -236,6 +237,16 @@ enum sbi_pmu_ctr_type {
 /* Flags defined for counter stop function */
 #define SBI_PMU_STOP_FLAG_RESET (1 << 0)
 
+/* SBI function IDs for FW feature extension */
+#define SBI_EXT_FW_FEATURE_SET		0x0
+#define SBI_EXT_FW_FEATURE_GET		0x1
+
+enum sbi_fw_features_t {
+	SBI_FW_FEATURE_MISALIGNED_DELEG	= 0,
+
+	SBI_FW_FEATURE_MAX,
+};
+
 #define SBI_SPEC_VERSION_DEFAULT	0x1
 #define SBI_SPEC_VERSION_MAJOR_SHIFT	24
 #define SBI_SPEC_VERSION_MAJOR_MASK	0x7f
diff --git a/arch/riscv/kernel/sbi.c b/arch/riscv/kernel/sbi.c
index c672c8ba9a2a..3be48791455a 100644
--- a/arch/riscv/kernel/sbi.c
+++ b/arch/riscv/kernel/sbi.c
@@ -494,6 +494,16 @@ int sbi_remote_hfence_vvma_asid(const struct cpumask *cpu_mask,
 }
 EXPORT_SYMBOL(sbi_remote_hfence_vvma_asid);
 
+static int sbi_fw_feature_set(enum sbi_fw_features_t feature, bool set)
+{
+	struct sbiret ret;
+
+	ret = sbi_ecall(SBI_EXT_FW_FEATURE, SBI_EXT_FW_FEATURE_SET, feature,
+			set, 0, 0, 0, 0);
+
+	return sbi_err_map_linux_errno(ret.error);
+}
+
 static void sbi_srst_reset(unsigned long type, unsigned long reason)
 {
 	sbi_ecall(SBI_EXT_SRST, SBI_EXT_SRST_RESET, type, reason,
@@ -624,6 +634,17 @@ void __init sbi_init(void)
 			sbi_srst_reboot_nb.priority = 192;
 			register_restart_handler(&sbi_srst_reboot_nb);
 		}
+		/*
+		 * TODO: this will likely need to be updated when SBI extension
+		 * is ratified
+		 */
+		if ((sbi_spec_version >= sbi_mk_version(1, 0)) &&
+		    (sbi_probe_extension(SBI_EXT_FW_FEATURE) > 0)) {
+			pr_info("SBI FW_FEATURE extension detected\n");
+			/* Request misaligned handling delegation */
+			sbi_fw_feature_set(SBI_FW_FEATURE_MISALIGNED_DELEG,
+					   true);
+		}
 	} else {
 		__sbi_set_timer = __sbi_set_timer_v01;
 		__sbi_send_ipi	= __sbi_send_ipi_v01;
-- 
2.40.1