linux-kernel - Re: [RFC PATCH 1/4] powerpc: Optimize register usage for esr register

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Message-ID: <CAEVVKH8wjaR3JqgkWX=X6n00nmPEjcUCHU=_5w_xxjsAWHFcUg@mail.gmail.com>
Date:   Fri, 6 Aug 2021 21:22:52 +0800
From:   Xiongwei Song <sxwjean@...il.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Xiongwei Song <sxwjean@...com>,
        PowerPC <linuxppc-dev@...ts.ozlabs.org>, oleg@...hat.com,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, ravi.bangoria@...ux.ibm.com,
        npiggin@...il.com, aneesh.kumar@...ux.ibm.com,
        sandipan@...ux.ibm.com, efremov@...ux.com, peterx@...hat.com,
        akpm@...ux-foundation.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/4] powerpc: Optimize register usage for esr register

On Fri, Aug 6, 2021 at 3:32 PM Christophe Leroy
<christophe.leroy@...roup.eu> wrote:
>
>
>
> Le 06/08/2021 à 05:16, Xiongwei Song a écrit :
> > On Thu, Aug 5, 2021 at 6:06 PM Christophe Leroy
> > <christophe.leroy@...roup.eu> wrote:
> >>
> >>
> >>
> >> Le 26/07/2021 à 16:30, sxwjean@...com a écrit :
> >>> From: Xiongwei Song <sxwjean@...il.com>
> >>>
> >>> Create an anonymous union for dsisr and esr regsiters, we can reference
> >>> esr to get the exception detail when CONFIG_4xx=y or CONFIG_BOOKE=y.
> >>> Otherwise, reference dsisr. This makes code more clear.
> >>
> >> I'm not sure it is worth doing that.
> > Why don't we use "esr" as reference manauls mentioned?
> >
> >>
> >> What is the point in doing the following when you know that regs->esr and regs->dsisr are exactly
> >> the same:
> >>
> >>   > -    err = ___do_page_fault(regs, regs->dar, regs->dsisr);
> >>   > +    if (IS_ENABLED(CONFIG_4xx) || IS_ENABLED(CONFIG_BOOKE))
> >>   > +            err = ___do_page_fault(regs, regs->dar, regs->esr);
> >>   > +    else
> >>   > +            err = ___do_page_fault(regs, regs->dar, regs->dsisr);
> >>   > +
> > Yes, we can drop this. But it's a bit vague.
> >
> >> Or even
> >>
> >>   > -    int is_write = page_fault_is_write(regs->dsisr);
> >>   > +    unsigned long err_reg;
> >>   > +    int is_write;
> >>   > +
> >>   > +    if (IS_ENABLED(CONFIG_4xx) || IS_ENABLED(CONFIG_BOOKE))
> >>   > +            err_reg = regs->esr;
> >>   > +    else
> >>   > +            err_reg = regs->dsisr;
> >>   > +
> >>   > +    is_write = page_fault_is_write(err_reg);
> >>
> >>
> >> Artificially growing the code for that makes no sense to me.
> >
> > We can drop this too.
> >>
> >>
> >> To avoid anbiguity, maybe the best would be to rename regs->dsisr to something like regs->sr , so
> >> that we know it represents the status register, which is DSISR or ESR depending on the platform.
> >
> > If so, this would make other people more confused. My consideration is
> > to follow what the reference
> > manuals represent.
>
> Maybe then we could rename the fields as regs->dsisr_esr and regs->dar_dear

I still prefer my method.

>
> That would be more explicit for everyone.
>
> The UAPI header however should remain as is because anonymous unions are not supported by old
> compilers as mentioned by Michael.

Sure. Will update in v2.

>
> But nevertheless, there are also situations where was is stored in regs->dsisr is not what we have
> in DSISR register. For instance on an ISI exception, we store a subset of the content of SRR1
> register into regs->dsisr.

Can I think my method has better expansibility here;-)?
Let me finish esr and dear first. Thank you for the reminder.

Regards,
Xiongwei
>
> Christophe