Re: [PATCH v2] arm64: vdso32: add CONFIG_THUMB2_COMPAT_VDSO

From: Will Deacon <will@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Naohiro Aota <naohiro.aota@wdc.com>,
	Stephen Boyd <swboyd@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Manoj Gupta <manojgupta@google.com>,
	Luis Lozano <llozano@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v2] arm64: vdso32: add CONFIG_THUMB2_COMPAT_VDSO
Date: Wed, 10 Jun 2020 09:47:55 +0100	[thread overview]
Message-ID: <20200610084755.GB17035@willie-the-truck> (raw)
In-Reply-To: <CAKwvOdnDeYQ-tbyW8=tcKcbbFxtDzqk8dCUSzbBYP1amb9a2hg@mail.gmail.com>

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@arm.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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel