Re: [Minios-devel] [PATCH 37/40] arm64: add the makefile

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Steve,

On 08/11/17 14:44, Steve Capper wrote:
> On Tue, Nov 07, 2017 at 11:56:16AM +0000, Julien Grall wrote:
> > Hi Steve,
> >
> > On 07/11/17 09:34, Steve Capper wrote:
> > > On Tue, Nov 07, 2017 at 04:32:36PM +0800, Huang Shijie wrote:
> > > > On Mon, Nov 06, 2017 at 06:00:30PM +0000, Julien Grall wrote:
> > > > > Hi Shijie,
> > > > >
> > > > > On 03/11/17 03:12, Huang Shijie wrote:
> [...]
>
> > > > > > +ifeq ($(MINIOS_TARGET_ARCH),arm64)
> > > > > > +ARCH_CFLAGS  := -D__aarch64__ -DXEN_HAVE_PV_GUEST_ENTRY -mgeneral-regs-only 
> > > > > > -fno-PIE
> > > > > Why do you need to define __aarch64__? The compiler should to it for you.
> > > > Yes. we can remove it.
> > > > > Why do you define XEN_HAVE_PV_GUEST_ENTRY? Arm does not have any start_info
> > > > > page.
> > > > okay, I will remove.
> > > >
> > > > > Lastly, why -fno-PIE?
> > > > Steve tested a platform which compile very slowly, and need the -fno-PIE
> > > > for it.
> > > Hi,
> > >
> > > -fno-PIE is needed when building with a Debian Stretch or newer toolchain,
> > > as these are configured with: --enable-default-pie
> > >
> > > If we omit -fno-PIE then a global object table is used where we are not
> > > expecting it and this results in crashes in the early code as uninitialised
> > > pointers are dereferenced.
> > Thank you for the explanation! But I am slightly confused, I tried to build
> > Xen for Arm64 (AFAICT we don't set fno-pie) the GCC from Stretch. I see no
> > issue to boot it.
> >
> > So how is that different with Mini-OS?
> Hi Julien,
> This was tricky to find :-), I think the following in Xen accomplishes the same
> result in include/xen/compiler.h:
>
> #ifdef GCC_HAS_VISIBILITY_ATTRIBUTE
> /* Results in more efficient PIC code (no indirections through GOT or PLT). */
> #pragma GCC visibility push(hidden)
> #endif
>
> One can see if global object table relocations are present in a .o file via:
> objdump -dr built-in.o
>
> I can't see these in Xen binaries I build for arm64. If we omit the -fno-PIE
> in Mini-OS, we get something like:
> 45e4:       90000000        adrp    x0, 8 <_start-0x38>
> 45e4: R_AARCH64_ADR_GOT_PAGE    physical_address_offset
Thank you for the explanation :).

> I think we may need a similar fix for x86 in mini-os too.
I think you are right. Wei, Samuel, Juergen?

Cheers,

--
Julien Grall

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