RISC-V: Implement modular CSR helper interface

Previous CSR code uses csr_read_helper and csr_write_helper
to update CSR registers however this interface prevents
atomic read/modify/write CSR operations; in addition
there is no trap-free method to access to CSRs due
to the monolithic CSR functions call longjmp.

The current iCSR interface is not safe to be called by
target/riscv/gdbstub.c as privilege checks or missing CSRs
may call longjmp to generate exceptions. It needs to
indicate existence so traps can be generated in the
CSR instruction helpers.

This commit moves CSR access from the monolithic switch
statements in target/riscv/op_helper.c into modular
read/write functions in target/riscv/csr.c using a new
function pointer table for dispatch (which can later
be used to allow CPUs to hook up model specific CSRs).

A read/modify/write interface is added to support atomic
CSR operations and a non-trapping interface is added
to allow exception-free access to CSRs by the debugger.

The CSR functions and CSR dispatch table are ordered
to match The RISC-V Instruction Set Manual, Volume II:
Privileged Architecture Version 1.10, 2.2 CSR Listing.

An API is added to allow derived cpu instances to modify
or implement new CSR operations.

Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/target/riscv/gdbstub.c b/target/riscv/gdbstub.c
index 4f919b6..3cabb21 100644
--- a/target/riscv/gdbstub.c
+++ b/target/riscv/gdbstub.c
@@ -33,7 +33,10 @@ int riscv_cpu_gdb_read_register(CPUState *cs, uint8_t *mem_buf, int n)
     } else if (n < 65) {
         return gdb_get_reg64(mem_buf, env->fpr[n - 33]);
     } else if (n < 4096 + 65) {
-        return gdb_get_regl(mem_buf, csr_read_helper(env, n - 65));
+        target_ulong val = 0;
+        if (riscv_csrrw(env, n - 65, &val, 0, 0) == 0) {
+            return gdb_get_regl(mem_buf, val);
+        }
     }
     return 0;
 }
@@ -56,7 +59,10 @@ int riscv_cpu_gdb_write_register(CPUState *cs, uint8_t *mem_buf, int n)
         env->fpr[n - 33] = ldq_p(mem_buf); /* always 64-bit */
         return sizeof(uint64_t);
     } else if (n < 4096 + 65) {
-        csr_write_helper(env, ldtul_p(mem_buf), n - 65);
+        target_ulong val = ldtul_p(mem_buf);
+        if (riscv_csrrw(env, n - 65, NULL, val, -1) == 0) {
+            return sizeof(target_ulong);
+        }
     }
     return 0;
 }