Re: [PATCH 1/2] target/riscv: Fix the mstatus.MPP value after executing MRET

[liweiwei]

On 2023/4/6 08:43, Alistair Francis wrote:

> On Thu, Mar 30, 2023 at 11:59 PM Weiwei Li <liweiwei@iscas.ac.cn> wrote:
> > The MPP will be set to the least-privileged supported mode (U if
> > U-mode is implemented, else M).
> >       
> I don't think this is right, the spec in section 8.6.4 says this:
>

Sorry, I didn't find this section in latest release of both privilege and un-privilege spec

(draft-20230131-c0b298a: Clarify WFI trapping behavior (#972)).

> "MRET then in mstatus/mstatush sets MPV=0, MPP=0,
> MIE=MPIE, and MPIE=1"
>

In section 3.1.6.1, the privilege spec says this:

"An MRET or SRET instruction is used to return from a trap in M-mode or S-mode respectively.
When executing an xRET instruction, supposing xPP holds the value y, xIE is set to xPIE; the
privilege mode is changed to y; xPIE is set to 1; and xPP is set to the least-privileged supported
mode (U if U-mode is implemented, else M). If y̸=M, xRET also sets MPRV=0"

And I think PRV_U is an illegal value for MPP if U-mode is not implemented.

Regards,

Weiwei Li

> So it should just always be 0 (PRV_U is 0)
>
> Alistair
>
> > Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
> > Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
> > ---
> >  target/riscv/op_helper.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c
> > index 84ee018f7d..991f06d98d 100644
> > --- a/target/riscv/op_helper.c
> > +++ b/target/riscv/op_helper.c
> > @@ -339,7 +339,8 @@ target_ulong helper_mret(CPURISCVState *env)
> >      mstatus = set_field(mstatus, MSTATUS_MIE,
> >                          get_field(mstatus, MSTATUS_MPIE));
> >      mstatus = set_field(mstatus, MSTATUS_MPIE, 1);
> > -    mstatus = set_field(mstatus, MSTATUS_MPP, PRV_U);
> > +    mstatus = set_field(mstatus, MSTATUS_MPP,
> > +                        riscv_has_ext(env, RVU) ? PRV_U : PRV_M);
> >      mstatus = set_field(mstatus, MSTATUS_MPV, 0);
> >      if ((env->priv_ver >= PRIV_VERSION_1_12_0) && (prev_priv != PRV_M)) {
> >          mstatus = set_field(mstatus, MSTATUS_MPRV, 0);
> > --
> > 2.25.1
> >
> >
> >       
>