[RFC PATCH v2 4/8] arm64: compat: Add a 32-bit vDSO

[Catalin Marinas]

On Fri, Oct 28, 2016 at 11:20:07AM +0100, Kevin Brodsky wrote:
> On 28/10/16 04:09, Jisheng Zhang wrote:
> > On Thu, 27 Oct 2016 17:30:54 +0100 Kevin Brodsky wrote:
> > > Provide the files necessary for building a compat (AArch32) vDSO in
> > > kernel/vdso32.
> > > 
> > > This is mostly an adaptation of the arm vDSO. The most significant
> > > change in vgettimeofday.c is the use of the arm64 vdso_data struct,
> > > allowing the vDSO data page to be shared between the 32 and 64-bit
> > > vDSOs.
> > > 
> > > In addition to the time functions, sigreturn trampolines are also
> > > provided, aiming at replacing those in the vector page. To improve
> > > debugging, CFI and unwinding directives are used, based on glibc's
> > > implementation. Symbol offsets are made available to the kernel using
> > > the same method as the 64-bit vDSO.
> > > 
> > > There is unfortunately an important caveat to all this: we cannot get
> > > away with hand-coding 32-bit instructions like in kernel/kuser32.S,
> > > this time we really need a 32-bit compiler. The compat vDSO Makefile
> > > relies on CROSS_COMPILE_ARM32 to provide a 32-bit compiler,
> > > appropriate logic will be added to the arm64 Makefile later on to
> > > ensure that an attempt to build the compat vDSO is made only if this
> > > variable has been set properly.
> > > 
> > > Signed-off-by: Kevin Brodsky <kevin.brodsky at arm.com>
> > > ---
> > >   arch/arm64/kernel/vdso32/Makefile        | 121 +++++++++++++
> > >   arch/arm64/kernel/vdso32/sigreturn.S     |  86 +++++++++
> > >   arch/arm64/kernel/vdso32/vdso.S          |  32 ++++
> > >   arch/arm64/kernel/vdso32/vdso.lds.S      |  98 +++++++++++
> > >   arch/arm64/kernel/vdso32/vgettimeofday.c | 294 +++++++++++++++++++++++++++++++
> > >   5 files changed, 631 insertions(+)
> > >   create mode 100644 arch/arm64/kernel/vdso32/Makefile
> > >   create mode 100644 arch/arm64/kernel/vdso32/sigreturn.S
> > >   create mode 100644 arch/arm64/kernel/vdso32/vdso.S
> > >   create mode 100644 arch/arm64/kernel/vdso32/vdso.lds.S
> > >   create mode 100644 arch/arm64/kernel/vdso32/vgettimeofday.c
> > > 
> > > diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> > > new file mode 100644
> > > index 000000000000..38facc870f6e
> > > --- /dev/null
> > > +++ b/arch/arm64/kernel/vdso32/Makefile
> > > @@ -0,0 +1,121 @@
> > > +#
> > > +# Building a vDSO image for AArch32.
> > > +#
> > > +# Author: Kevin Brodsky <kevin.brodsky at arm.com>
> > > +# A mix between the arm64 and arm vDSO Makefiles.
> > > +
> > > +CC_ARM32 := $(CROSS_COMPILE_ARM32)gcc
> > > +
> > > +# Same as cc-ldoption, but using CC_ARM32 instead of CC
> > > +cc32-ldoption = $(call try-run,\
> > > +        $(CC_ARM32) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
> > > +
> > > +# Borrow vdsomunge.c from the arm vDSO
> > > +munge := arch/arm/vdso/vdsomunge
> > > +hostprogs-y := $(srctree)/$(munge)
> > > +
> > > +c-obj-vdso := vgettimeofday.o
> > > +asm-obj-vdso := sigreturn.o
> > > +
> > > +# Build rules
> > > +targets := $(c-obj-vdso) $(asm-obj-vdso) vdso.so vdso.so.dbg vdso.so.raw
> > > +c-obj-vdso := $(addprefix $(obj)/, $(c-obj-vdso))
> > > +asm-obj-vdso := $(addprefix $(obj)/, $(asm-obj-vdso))
> > > +obj-vdso := $(c-obj-vdso) $(asm-obj-vdso)
> > > +
> > > +ccflags-y := -fPIC -fno-common -fno-builtin -fno-stack-protector
> > > +ccflags-y += -DDISABLE_BRANCH_PROFILING
> > > +
> > > +# Force -O2 to avoid libgcc dependencies
> > > +VDSO_CFLAGS := -march=armv8-a -O2
> > For completeness, bringing 32bit compiler need to check whether the 32bit
> > toolchain support some options. IIRC, armv8-a support isn't enabled until
> > gcc 4.8, so old toolchains such gcc-4.7 will complain:
> >   error: unrecognized argument in option ‘-march=armv8-a’
> 
> That's a fair point. I guess -march=armv8-a is not strictly necessary and
> the produced vDSO should be fine if arch/arm/vdso also compiles fine.
> However we would still need to pass -march=armv7-a. I'm not sure what to do
> between:
> * Checking that the compiler supports -march=armv8-a when inspecting
> CROSS_COMPILE_ARM32, and if it doesn't vdso32 will not be built.
> * Checking whether -march=armv8-a is available here, and if it is not fall
> back to -march=armv7-a.

Does v8 vs v7 make any difference in the generated code? If not, we
could just stick to armv7-a permanently.

-- 
Catalin