Store user-stack-pointer on kernel stack when pushing/popping GPRs

The current implementation for pushing/popping GPRs stores the USP in
xSCRATCH, however this CSR is also used for a counter value by the
code section inserted by (cfg.enable_nested_interrupt) within
riscv_asm_program_gen::gen_interrupt_handler_section(). This corrupts the
program state because the user-program GPRs can no longer be restored.

Instead, push the USP onto the kernel stack before pushing the GPRs.
Pop it off again after popping the GPRs themselves.

src/riscv_instr_pkg.sv

@@ -1379,18 +1379,18 @@ package riscv_instr_pkg;
                                                    ref string instr[$]);
     string store_instr = (XLEN == 32) ? "sw" : "sd";
     if (scratch inside {implemented_csr}) begin
-      // Use kernal stack for handling exceptions
-      // Save the user mode stack pointer to the scratch register
-      instr.push_back($sformatf("csrrw x%0d, 0x%0x, x%0d", sp, scratch, sp));
-      // Move TP to SP
+      // Push USP from gpr.SP onto the kernel stack
+      instr.push_back($sformatf("addi x%0d, x%0d, -4", tp, tp));
+      instr.push_back($sformatf("%0s  x%0d, (x%0d)", store_instr, sp, tp));
+      // Move KSP to gpr.SP
       instr.push_back($sformatf("add x%0d, x%0d, zero", sp, tp));
     end
     // If MPRV is set and MPP is S/U mode, it means the address translation and memory protection
     // for load/store instruction is the same as the mode indicated by MPP. In this case, we
     // need to use the virtual address to access the kernel stack.
     if((status == MSTATUS) && (SATP_MODE != BARE)) begin
-      // We temporarily use tp to check mstatus to avoid changing other GPR. The value of sp has
-      // been saved to xScratch and can be restored later.
+      // We temporarily use tp to check mstatus to avoid changing other GPR.
+      // (The value of sp has been pushed to the kernel stack, so can be recovered later)
       if(mprv) begin
         instr.push_back($sformatf("csrr x%0d, 0x%0x // MSTATUS", tp, status));
         instr.push_back($sformatf("srli x%0d, x%0d, 11", tp, tp));  // Move MPP to bit 0
@@ -1401,14 +1401,18 @@ package riscv_instr_pkg;
         // Use virtual address for stack pointer
         instr.push_back($sformatf("slli x%0d, x%0d, %0d", sp, sp, XLEN - MAX_USED_VADDR_BITS));
         instr.push_back($sformatf("srli x%0d, x%0d, %0d", sp, sp, XLEN - MAX_USED_VADDR_BITS));
+        instr.push_back("1: nop");
       end
     end
-    // Reserve space from kernel stack to save all 32 GPR except for x0
-    instr.push_back($sformatf("1: addi x%0d, x%0d, -%0d", sp, sp, 31 * (XLEN/8)));
-    // Push all GPRs to kernel stack
+    // Push all GPRs (except for x0) to kernel stack
+    // (gpr.SP currently holds the KSP)
+    instr.push_back($sformatf("addi x%0d, x%0d, -%0d", sp, sp, 31 * (XLEN/8)));
     for(int i = 1; i < 32; i++) begin
       instr.push_back($sformatf("%0s  x%0d, %0d(x%0d)", store_instr, i, i * (XLEN/8), sp));
     end
+    // Move KSP back to gpr.TP
+    // (this is needed if we again take a interrupt (nested) before restoring our USP)
+    instr.push_back($sformatf("add x%0d, x%0d, zero", tp, sp));
   endfunction
 
   // Pop general purpose register from stack, this is needed before returning to user program
@@ -1419,17 +1423,19 @@ package riscv_instr_pkg;
                                                     riscv_reg_t tp,
                                                     ref string instr[$]);
     string load_instr = (XLEN == 32) ? "lw" : "ld";
-    // Pop user mode GPRs from kernel stack
+    // Move KSP to gpr.SP
+    instr.push_back($sformatf("add x%0d, x%0d, zero", sp, tp));
+    // Pop GPRs from kernel stack
     for(int i = 1; i < 32; i++) begin
       instr.push_back($sformatf("%0s  x%0d, %0d(x%0d)", load_instr, i, i * (XLEN/8), sp));
     end
-    // Restore kernel stack pointer
     instr.push_back($sformatf("addi x%0d, x%0d, %0d", sp, sp, 31 * (XLEN/8)));
     if (scratch inside {implemented_csr}) begin
-      // Move SP to TP
+      // Move KSP back to gpr.TP
       instr.push_back($sformatf("add x%0d, x%0d, zero", tp, sp));
-      // Restore user mode stack pointer
-      instr.push_back($sformatf("csrrw x%0d, 0x%0x, x%0d", sp, scratch, sp));
+      // Pop USP from the kernel stack, move back to gpr.SP
+      instr.push_back($sformatf("%0s  x%0d, (x%0d)", load_instr, sp, tp));
+      instr.push_back($sformatf("addi x%0d, x%0d, 4", tp, tp));
     end
   endfunction