RISC-V mstatus TSR bit not correctly implemented

Hi,

since qemu 4.1.0 the TSR bit in mstatus register is supported. But it does not allow for executing sret in m-mode.

From the RISC-V specifications:
"When TSR=1, attempts to execute SRET while executing in S-mode will raise an illegal instruction
exception. When TSR=0, this operation is permitted in S-mode."

This means an exception should only be raised when executing in S-mode, but not in M-mode, hence you should change the condition in helper_sret (target/riscv/op_helper.c) from:
     if (env->priv_ver >= PRIV_VERSION_1_10_0 &&
          get_field(env->mstatus, MSTATUS_TSR))
to:
     if (env->priv_ver >= PRIV_VERSION_1_10_0 &&
          get_field(env->mstatus, MSTATUS_TSR) && !(env->priv >= PRV_M))