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

[Alistair Francis]

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.

>
> 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

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
> >>
> >>
>