[PATCH v6 1/9] target/riscv: turn write_misa() into an official no-op

[Daniel Henrique Barboza]

At this moment, and apparently since ever, we have no way of enabling
RISCV_FEATURE_MISA. This means that all the code from write_misa(), all
the nuts and bolts that handles how to write this CSR, has always been a
no-op as well because write_misa() will always exit earlier.

This seems to be benign in the majority of cases. Booting an Ubuntu
'virt' guest and logging all the calls to 'write_misa' shows that no
writes to MISA CSR was attempted. Writing MISA, i.e. enabling/disabling
RISC-V extensions after the machine is powered on, seems to be a niche
use.

Before proceeding, let's recap what the spec says about MISA. It is a
CSR that is divided in 3 fields:

- MXL, Machine XLEN, described as "may be writable";

- MXLEN, the XLEN in M-mode, which is given by the setting of MXL or a
fixed value if MISA is zero;

- Extensions is defined as "a WARL field that can contain writable bits
where the implementation allows the supported ISA to be modified"

Thus what we have today (write_misa() being a no-op) is already a valid
spec implementation. We're not obliged to have a particular set of MISA
writable bits, and at this moment we have none.

Given that allowing the dormant code to write MISA can cause tricky bugs
to solve later on, and we don't have a particularly interesting case of
writing MISA to support today, and we're already not violating the
specification, let's erase all the body of write_misa() and turn it into
an official no-op instead of an accidental one. We'll keep consistent
with what we provide users today but with 50+ less lines to maintain.

RISCV_FEATURE_MISA enum is erased in the process since there's no one
else using it.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
---
 target/riscv/cpu.h |  1 -
 target/riscv/csr.c | 55 ----------------------------------------------
 2 files changed, 56 deletions(-)

diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
index 7128438d8e..01803a020d 100644
--- a/target/riscv/cpu.h
+++ b/target/riscv/cpu.h
@@ -89,7 +89,6 @@ enum {
     RISCV_FEATURE_MMU,
     RISCV_FEATURE_PMP,
     RISCV_FEATURE_EPMP,
-    RISCV_FEATURE_MISA,
     RISCV_FEATURE_DEBUG
 };
 
diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index 1b0a0c1693..f7862ff4a4 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -1329,61 +1329,6 @@ static RISCVException read_misa(CPURISCVState *env, int 
csrno,
 static RISCVException write_misa(CPURISCVState *env, int csrno,
                                  target_ulong val)
 {
-    if (!riscv_feature(env, RISCV_FEATURE_MISA)) {
-        /* drop write to misa */
-        return RISCV_EXCP_NONE;
-    }
-
-    /* 'I' or 'E' must be present */
-    if (!(val & (RVI | RVE))) {
-        /* It is not, drop write to misa */
-        return RISCV_EXCP_NONE;
-    }
-
-    /* 'E' excludes all other extensions */
-    if (val & RVE) {
-        /* when we support 'E' we can do "val = RVE;" however
-         * for now we just drop writes if 'E' is present.
-         */
-        return RISCV_EXCP_NONE;
-    }
-
-    /*
-     * misa.MXL writes are not supported by QEMU.
-     * Drop writes to those bits.
-     */
-
-    /* Mask extensions that are not supported by this hart */
-    val &= env->misa_ext_mask;
-
-    /* Mask extensions that are not supported by QEMU */
-    val &= (RVI | RVE | RVM | RVA | RVF | RVD | RVC | RVS | RVU | RVV);
-
-    /* 'D' depends on 'F', so clear 'D' if 'F' is not present */
-    if ((val & RVD) && !(val & RVF)) {
-        val &= ~RVD;
-    }
-
-    /* Suppress 'C' if next instruction is not aligned
-     * TODO: this should check next_pc
-     */
-    if ((val & RVC) && (GETPC() & ~3) != 0) {
-        val &= ~RVC;
-    }
-
-    /* If nothing changed, do nothing. */
-    if (val == env->misa_ext) {
-        return RISCV_EXCP_NONE;
-    }
-
-    if (!(val & RVF)) {
-        env->mstatus &= ~MSTATUS_FS;
-    }
-
-    /* flush translation cache */
-    tb_flush(env_cpu(env));
-    env->misa_ext = val;
-    env->xl = riscv_cpu_mxl(env);
     return RISCV_EXCP_NONE;
 }
 
-- 
2.39.1