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

[liweiwei]

On 2023/4/6 10:24, Alistair Francis wrote:
> On Thu, Apr 6, 2023 at 12:14 PM liweiwei <liweiwei@iscas.ac.cn> wrote:
> > 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.
> They seem to be in conflict. It's probably worth opening an issue
> against the spec to get some clarification here.
OK. I'll send an issue for it.
> > And MPP is WARL field.  PRV_U will be an illegal value for MPP if U-mode
> > is not implemented.
> Yeah, I think you are right. It just directly goes against the mret
> section. I suspect the mret section is wrong and needs to be updated
>
> > 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
> Have a look at your previous email, it's a HTML email. If I view the
> source of the email I see this:
>
>      Content-Type: text/html; charset=UTF-8
>
> and the formatting is a little off.
>
> This email that I'm replying to is a plain text email. I'm not sure
> what happened, but try to check that your responses are plain text. I
> think there is a setting in Thunderbird to just open and reply to all
> emails as plain text, which is probably worth turning on

OK . Thanks! I'll try to set it later.

Regards,

Weiwei Li

> Alistair
>
> > 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