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

[liweiwei]

On 2023/4/6 09:46, Alistair Francis wrote:
> On Thu, Apr 6, 2023 at 10:56 AM liweiwei <liweiwei@iscas.ac.cn> wrote:
> > 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
> I updated my spec, using commit
> f6b8d5c7d2dcd935b48689a337c8f5bc2be4b5e5 it's now section 9.6.4 Trap
> Return

Yeah. I see it. However, this is a little different from the description 
in section 3.1.6.1.

And MPP is WARL field.  PRV_U will be an illegal value for MPP if U-mode 
is not implemented.

So I think description in section 3.1.6.1 seems more reasonable.

> > (draft-20230131-c0b298a: Clarify WFI trapping behavior (#972)).
> Also, you replied with a HTML email which loses the conversation
> history (just see above). Can you fixup your client to reply with
> plain text please

Sorry. I don't get your problem. I replied by Thunderbird. Above is the 
title for the latest release version of the spec in riscv-isa-manual 
github 
(https://github.com/riscv/riscv-isa-manual/releases/tag/draft-20230131-c0b298a).

Regards,

Weiwei Li

> Alistair
>
> > "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