[Bug 1851939] [NEW] RISC-V mstatus TSR bit not correctly implemented

[tm]

Public bug reported:

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))

** Affects: qemu
     Importance: Undecided
         Status: New


** Tags: riscv

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https://bugs.launchpad.net/bugs/1851939

Title:
  RISC-V mstatus TSR bit not correctly implemented

Status in QEMU:
  New

Bug description:
  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))

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