[PATCH v7 03/10] target/riscv: allow MISA writes as experimental

[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 properly 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.

After discussions in the mailing list, most notably in [1], we reached
the consensus that this code is not suited to be exposed to users
because it's not well tested, but at the same time removing it is a bit
extreme because we would like to fix it, and it's easier to do so with
the code available to use instead of fetching it from git log.

The approach taken here is to get rid of RISCV_FEATURE_MISA altogether
and use a new experimental flag called x-misa-w. The default value is
false, meaning that we're keeping the existing behavior of doing nothing
if a write_misa() is attempted. As with any existing experimental flag,
x-misa-w is also a temporary flag that we need to remove once we fix
write_misa().

[1] https://lists.gnu.org/archive/html/qemu-devel/2023-02/msg05092.html

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
---
 target/riscv/cpu.c | 6 ++++++
 target/riscv/cpu.h | 2 +-
 target/riscv/csr.c | 2 +-
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index 93b52b826c..1d637b1acd 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -1210,6 +1210,12 @@ static Property riscv_cpu_properties[] = {
 
     DEFINE_PROP_BOOL("rvv_ta_all_1s", RISCVCPU, cfg.rvv_ta_all_1s, false),
     DEFINE_PROP_BOOL("rvv_ma_all_1s", RISCVCPU, cfg.rvv_ma_all_1s, false),
+
+    /*
+     * write_misa() is marked as experimental for now so mark
+     * it with -x and default to 'false'.
+     */
+    DEFINE_PROP_BOOL("x-misa-w", RISCVCPU, cfg.misa_w, false),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
index 215423499e..9d3304bcda 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
 };
 
@@ -498,6 +497,7 @@ struct RISCVCPUConfig {
     bool pmp;
     bool epmp;
     bool debug;
+    bool misa_w;
 
     bool short_isa_string;
 };
diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index e149b453da..3cb8d2ffad 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -1329,7 +1329,7 @@ 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)) {
+    if (!riscv_cpu_cfg(env)->misa_w) {
         /* drop write to misa */
         return RISCV_EXCP_NONE;
     }
-- 
2.39.2