[RFC PATCH 2/7] target/riscv: Add SSTATUS_UBE

[Roan Richmond]

From: Lawrence Hunter <lawrence.hunter@codethink.co.uk>

Adds the UBE flag for SSTATUS this a read-only mirror of
MSTATUS UBE.

Authored-by: Lawrence Hunter <lawrence.hunter@codethink.co.uk>
Co-authored-by: Ben Dooks <ben.dooks@codethink.co.uk>
Co-authored-by: Roan Richmond <roan.richmond@codethink.co.uk>
Signed-off-by: Lawrence Hunter <lawrence.hunter@codethink.co.uk>
---
 target/riscv/cpu_bits.h | 1 +
 target/riscv/csr.c      | 6 ++++--
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/target/riscv/cpu_bits.h b/target/riscv/cpu_bits.h
index 2cc12a06b6..6d8e4eaf32 100644
--- a/target/riscv/cpu_bits.h
+++ b/target/riscv/cpu_bits.h
@@ -583,6 +583,7 @@ typedef enum {
 #define SSTATUS_SIE         0x00000002
 #define SSTATUS_UPIE        0x00000010
 #define SSTATUS_SPIE        0x00000020
+#define SSTATUS_UBE         0x00000040
 #define SSTATUS_SPP         0x00000100
 #define SSTATUS_VS          0x00000600
 #define SSTATUS_FS          0x00006000
diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index 0ee4614bf1..5e7933dea0 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -2980,7 +2980,8 @@ static RISCVException read_sstatus_i128(CPURISCVState 
*env, int csrno,
 static RISCVException read_sstatus(CPURISCVState *env, int csrno,
                                    target_ulong *val)
 {
-    target_ulong mask = (sstatus_v1_10_mask);
+    // Allow reading from SSTATUS_UBE
+    target_ulong mask = sstatus_v1_10_mask | SSTATUS_UBE;
     if (env->xl != MXL_RV32 || env->debugger) {
         mask |= SSTATUS64_UXL;
     }
@@ -2997,7 +2998,8 @@ static RISCVException read_sstatus(CPURISCVState *env, 
int csrno,
 static RISCVException write_sstatus(CPURISCVState *env, int csrno,
                                     target_ulong val)
 {
-    target_ulong mask = (sstatus_v1_10_mask);
+    // Disallow writing to SSTATUS_UBE as read-only
+    target_ulong mask = (sstatus_v1_10_mask) & (~SSTATUS_UBE);
 
     if (env->xl != MXL_RV32 || env->debugger) {
         if ((val & SSTATUS64_UXL) != 0) {
-- 
2.43.0