Re: [PATCH v3 3/4] RISC-V: Support the read-only CSR checking.

[Jim Wilson <jimw at sifive dot com>]

On Sun, Dec 15, 2019 at 9:22 PM Nelson Chu <nelson.chu@sifive.com> wrote:
>         gas/
>         * config/tc-riscv.c (insn_with_csr): New boolean to indicate we are
>         assembling instruction with CSR.
>         (enum csr_insn_type): New enum is used to classify the CSR instruction.
>         (riscv_csr_insn_type, riscv_csr_read_only_check): New functions.  These
>         are used to check if we write a read-only CSR by the CSR instruction.
>         (riscv_ip): Call riscv_csr_read_only_check after parsing all arguments.
>         * testsuite/gas/riscv/priv-reg-fail-read-only-01.s: New testcase.  Test
>         all CSR for the read-only CSR checking.
>         * testsuite/gas/riscv/priv-reg-fail-read-only-01.d: Likewise.
>         * testsuite/gas/riscv/priv-reg-fail-read-only-01.l: Likewise.
>         * testsuite/gas/riscv/priv-reg-fail-read-only-02.s: New testcase.  Test
>         all CSR instructions for the read-only CSR checking.
>         * testsuite/gas/riscv/priv-reg-fail-read-only-02.d: Likewise.
>         * testsuite/gas/riscv/priv-reg-fail-read-only-02.l: Likewise.

This makes insn_with_csr a global variable.  It looks like it should
be local to riscv_ip.

The testcases are using 1.12 draft csrs, which makes it dependent on
the first patch in the series.

> +                     error = _("Read-only CSR is used");

The error looks a little odd to me.  Using implies that you are
reading it, which is OK for a read-only CSR.  I would suggest "written
to" instead of "used".  That makes the error more obvious.

Also, maybe this should be a warning instead of an error.  This is a
valid instruction after all, it is just one that will always give a
run-time trap.  But it is possible that people might be writing code
this way for a reason, for instance, maybe a hardware verification
testsuite to test whether the instruction does give a trap exactly as
it is supposed to.

There is a function without an explanatory comment before it which is
a minor issue.

Otherwise this looks OK.

Jim