[PATCH for-8.1 16/17] target/riscv: do not allow RVG in write_misa()

[Daniel Henrique Barboza <dbarboza@ventanamicro.com>]

We're getting ready to use riscv_cpu_validate_set_extensions() to unify
the handling of write_misa() with the rest of the code base. But first
we need to deal with RVG.

The 'G' virtual extension enables a set of extensions in the CPU. At
this moment, this is done at the start of our validation step in
riscv_cpu_validate_set_extensions(). This means that enabling G will
enable other extensions in the CPU before resuming the validation.

This also means that, in case a write_misa() validation fails, we're
going to set cpu->cfg attributes that are unrelated to misa_ext bits
(icsr and ifencei). These would be 2 extra states that we would need to
store to fallback from a validation failure.

Since write_misa() is still on experimental state let's make our lives
easier for now and disable RVG updates.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
---
 target/riscv/csr.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index ab566639e5..02a5c2a5ca 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -1347,6 +1347,11 @@ static RISCVException write_misa(CPURISCVState *env, int csrno,
         return RISCV_EXCP_NONE;
     }
 
+    /* Changing 'G' state is unsupported */
+    if (val & RVG) {
+        return RISCV_EXCP_NONE;
+    }
+
     /* 'I' or 'E' must be present */
     if (!(val & (RVI | RVE))) {
         /* It is not, drop write to misa */
-- 
2.39.2