linux-kernel - Re: Getting the instruction pointer on a per arch basis

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Message-ID: <CAKwvOdnAzsF_vXH4UQiQd9HwjUju_sU9R4523zxqJSphuut-7Q@mail.gmail.com>
Date:   Wed, 1 Aug 2018 11:21:13 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     schwidefsky@...ibm.com
Cc:     heiko.carstens@...ibm.com,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>, dalias@...c.org,
        tony.luck@...el.com, fenghua.yu@...el.com, msalter@...hat.com,
        jacquiot.aurelien@...il.com,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        Pirama Arumuga Nainar <pirama@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Mark Salyzyn <salyzyn@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-c6x-dev@...ux-c6x.org, sparclinux@...r.kernel.org
Subject: Re: Getting the instruction pointer on a per arch basis

On Tue, Jul 31, 2018 at 10:41 PM Martin Schwidefsky
<schwidefsky@...ibm.com> wrote:
>
> On Tue, 31 Jul 2018 16:09:06 -0700
> Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> > + More maintainers and lists for visibility
> >
> > On Tue, Jul 31, 2018 at 3:32 PM Nick Desaulniers
> > <ndesaulniers@...gle.com> wrote:
> > >
> > > I'm currently looking into cleaning up the code duplication between
> > > current_text_addr() and _THIS_IP_, virtually every implementation of
> > > current_text_addr() and _THIS_IP_ itself are basically:
> > >
> > > #define _THIS_IP_ ({ __label__ _l; _l: &&_l; })
> > >
> > > For a few arch's, they have inline assembly instead (for
> > > current_text_addr()).  Examples:
> > > * s390
> > > * sh
> > > * ia64
> > > * x86 (um and 32b)
> > > * c6x
> > > * sparc
> > >
> > > I have a patch that cuts down on the duplication, but I don't
> > > understand why the few arch specific implementations are necessary.  I
> > > could reduce the duplication further if it's ok to just use the
> > > statement expression.
> > >
> > > Does anyone know why this is the case?
>
> For s390 it is just that we did not know about the label trick when we
> introduced the define. The inline has an advantage though, the code
> generated with the label trick is using a LARL instruction which is
> 4 bytes, the inline assembly uses a BASR which is 2 bytes.
>
> If I use the label method in current_text_addr() the size of vmlinux
> increases by a small amount:
>
> add/remove: 33/13 grow/shrink: 101/48 up/down: 11941/-8887 (3054)

Thanks for the measurements.  Was this output produced by a utility?

> This is acceptable though, I would not mind if _THIS_IP_ and
> current_text_addr use a common definition using labels.

Thank you for this feedback Martin, I appreciate it.  Patches soon.

-- 
Thanks,
~Nick Desaulniers