linux-kernel - Re: [PATCH v2] arm64: vdso32: add CONFIG_THUMB2_COMPAT

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Message-ID: <20200610084755.GB17035@willie-the-truck>
Date:   Wed, 10 Jun 2020 09:47:55 +0100
From:   Will Deacon <will@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Stephen Boyd <swboyd@...gle.com>,
        Robin Murphy <robin.murphy@....com>,
        Dave Martin <Dave.Martin@....com>,
        Luis Lozano <llozano@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Naohiro Aota <naohiro.aota@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2] arm64: vdso32: add CONFIG_THUMB2_COMPAT_VDSO

On Tue, Jun 09, 2020 at 04:55:13PM -0700, Nick Desaulniers wrote:
> On Tue, Jun 9, 2020 at 1:35 PM Catalin Marinas <catalin.marinas@....com> wrote:
> >
> > On Mon, Jun 08, 2020 at 01:57:08PM -0700, Nick Desaulniers wrote:
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index 7f9d38444d6d..fe9e6b231cac 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -1299,6 +1299,14 @@ config COMPAT_VDSO
> > >         You must have a 32-bit build of glibc 2.22 or later for programs
> > >         to seamlessly take advantage of this.
> > >
> > > +config THUMB2_COMPAT_VDSO
> > > +     bool "Compile the vDSO in THUMB2 mode"
> > > +     depends on COMPAT_VDSO
> > > +     default y
> > > +     help
> > > +       Compile the compat vDSO with -mthumb -fomit-frame-pointer if y, otherwise
> > > +       as -marm.
> >
> > Now that we understood the issue (I think), do we actually need this
> > choice? Why not going for -mthumb -fomit-frame-pointer always for the
> > compat vdso?
> 
> "Why should the compat vdso be configurable?" is a fair question.  I
> don't have an answer, but maybe some of the folks on thread do?
> 
> Our problem is more so "if the vdso is built as thumb, we need it also
> explicitly built with -fomit-frame-pointer."  Whether it should be
> built as thumb, arm, or configurable (and which default to pick in
> that case) are still an open questions.  Will asked for it to be
> configurable, so I sent a patch making it configurable.

It's configurable for 32-bit arm, so I was just following that as it's
hardly a maintenance burden to support both. I suppose you could have
a toolchain that only supports one or the other, but it does seem a little
esoteric if you're building a kernel for an arm64 CPU.

Will