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

[liweiwei <liweiwei@iscas.ac.cn>]

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
>>>>
>>>>