Fix VSSTATUS bits updation (#568)

This patch fixes VSSTATUS bits updations as follows:
1. set_csr() should not allow writs to VSSTATUS.SD bit instead
   VSSTATUS.SD bit should be derived again after VSSTATUS is updated
2. set_virt() should copy FS, VS and XS bits when transitioning
   from virt=on (VS/VU-mode) to virt=off (HS/M-mode)

Signed-off-by: Anup Patel <anup.patel@wdc.com>

riscv/encoding.h

@@ -50,7 +50,7 @@
 #define SSTATUS64_SD        0x8000000000000000
 
 #define SSTATUS_VS_MASK     (SSTATUS_SIE | SSTATUS_SPIE | \
-                             SSTATUS_SPP | SSTATUS_SUM | \
+                             SSTATUS_SPP | SSTATUS_FS | SSTATUS_SUM | \
                              SSTATUS_MXR | SSTATUS_UXL)
 
 #define HSTATUS_VSXL        0x300000000

riscv/processor.cc

@@ -630,21 +630,22 @@ void processor_t::set_virt(bool virt)
        * we should sync Guest/VM FS, VS, and XS state with Host FS,
        * VS, and XS state.
        */
-       if ((state.mstatus & SSTATUS_FS) == SSTATUS_FS) {
+      state.vsstatus &= ~SSTATUS_FS;
-         state.vsstatus |= SSTATUS_FS;
+      state.vsstatus |= (state.mstatus & SSTATUS_FS);
-         state.vsstatus |= (xlen == 64 ? SSTATUS64_SD : SSTATUS32_SD);
+      if (supports_extension('V')) {
-       }
+        state.vsstatus &= ~SSTATUS_VS;
-       if ((state.mstatus & SSTATUS_VS) == SSTATUS_VS) {
+        state.vsstatus |= (state.mstatus & SSTATUS_VS);
-         state.vsstatus |= SSTATUS_VS;
+      }
-         state.vsstatus |= (xlen == 64 ? SSTATUS64_SD : SSTATUS32_SD);
+      state.vsstatus &= ~SSTATUS_XS;
-       }
+      state.vsstatus |= (state.mstatus & SSTATUS_XS);
-       if ((state.mstatus & SSTATUS_XS) == SSTATUS_XS) {
+      state.vsstatus &= (xlen == 64 ? ~SSTATUS64_SD : ~SSTATUS32_SD);
-         state.vsstatus |= SSTATUS_XS;
+      if (((state.mstatus & SSTATUS_FS) == SSTATUS_FS) ||
+          ((state.vsstatus & SSTATUS_VS) == SSTATUS_VS) ||
+          ((state.vsstatus & SSTATUS_XS) == SSTATUS_XS)) {
          state.vsstatus |= (xlen == 64 ? SSTATUS64_SD : SSTATUS32_SD);
-       }
+      }
     }
     mask = SSTATUS_VS_MASK;
-    mask |= (supports_extension('F') ? SSTATUS_FS : 0);
     mask |= (supports_extension('V') ? SSTATUS_VS : 0);
     mask |= (xlen == 64 ? SSTATUS64_SD : SSTATUS32_SD);
     tmp = state.mstatus & mask;
@@ -1171,10 +1172,14 @@ void processor_t::set_csr(int which, reg_t val)
     }
     case CSR_VSSTATUS: {
       reg_t mask = SSTATUS_VS_MASK;
-      mask |= (supports_extension('F') ? SSTATUS_FS : 0);
       mask |= (supports_extension('V') ? SSTATUS_VS : 0);
-      mask |= (xlen == 64 ? SSTATUS64_SD : SSTATUS32_SD);
       state.vsstatus = (state.vsstatus & ~mask) | (val & mask);
+      state.vsstatus &= (xlen == 64 ? ~SSTATUS64_SD : ~SSTATUS32_SD);
+      if (((state.mstatus & SSTATUS_FS) == SSTATUS_FS) ||
+          ((state.vsstatus & SSTATUS_VS) == SSTATUS_VS) ||
+          ((state.vsstatus & SSTATUS_XS) == SSTATUS_XS)) {
+         state.vsstatus |= (xlen == 64 ? SSTATUS64_SD : SSTATUS32_SD);
+      }
       break;
     }
     case CSR_VSIE: {
@@ -1585,7 +1590,6 @@ reg_t processor_t::get_csr(int which, insn_t insn, bool write, bool peek)
     case CSR_HGEIP: ret(0);
     case CSR_VSSTATUS: {
       reg_t mask = SSTATUS_VS_MASK;
-      mask |= (supports_extension('F') ? SSTATUS_FS : 0);
       mask |= (supports_extension('V') ? SSTATUS_VS : 0);
       mask |= (xlen == 64 ? SSTATUS64_SD : SSTATUS32_SD);
       ret(state.vsstatus & mask);