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

Will Deacon
2020-06-10T08:48:03Z

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