linux-kernel - Re: [PATCH 3/8] x86/boot/64: Add support of additional page table level during early boot

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Date:   Tue, 11 Apr 2017 14:46:16 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...capital.net>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] x86/boot/64: Add support of additional page table
 level during early boot

On Tue, Apr 11, 2017 at 01:28:45PM +0200, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill@...temov.name> wrote:
> 
> > On Tue, Apr 11, 2017 at 09:02:03AM +0200, Ingo Molnar wrote:
> > > I realize that you had difficulties converting this to C, but it's not going to 
> > > get any easier in the future either, with one more paging mode/level added!
> > > 
> > > If you are stuck on where it breaks I'd suggest doing it gradually: first add a 
> > > trivial .c, build and link it in and call it separately. Then once that works, 
> > > move functionality from asm to C step by step and test it at every step.
> > 
> > I've described the specific issue with converting this code to C in cover
> > letter: how to make compiler to generate 32-bit code for a specific
> > function or translation unit, without breaking linking afterwards (-m32
> > break it).
> 
> Have you tried putting it into a separate .c file, and building it 32-bit?

Yes, I have. The patch below fails linking:

ld: i386 architecture of input file `arch/x86/boot/compressed/head64.o' is incompatible with i386:x86-64 output

> 
> I think arch/x86/entry/vdso/Makefile contains an example of how to build 32-bit 
> code even on 64-bit kernels.

I'll look closer (building proccess it's rather complicated), but my
understanding is that VDSO is stand-alone binary and doesn't really links
with the rest of the kernel, rather included as blob, no?

Andy, may be you have an idea?

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 44163e8c3868..8c1acacf408e 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -76,6 +76,8 @@ vmlinux-objs-$(CONFIG_EARLY_PRINTK) += $(obj)/early_serial_console.o
 vmlinux-objs-$(CONFIG_RANDOMIZE_BASE) += $(obj)/kaslr.o
 ifdef CONFIG_X86_64
 	vmlinux-objs-$(CONFIG_RANDOMIZE_BASE) += $(obj)/pagetable.o
+	vmlinux-objs-y += $(obj)/head64.o
+$(obj)/head64.o: KBUILD_CFLAGS := -m32 -D__KERNEL__ -O2
 endif
 
 $(obj)/eboot.o: KBUILD_CFLAGS += -fshort-wchar -mno-red-zone
diff --git a/arch/x86/boot/compressed/head64.c b/arch/x86/boot/compressed/head64.c
new file mode 100644
index 000000000000..42e1d64a15f4
--- /dev/null
+++ b/arch/x86/boot/compressed/head64.c
@@ -0,0 +1,3 @@
+void __startup32(void)
+{
+}
-- 
 Kirill A. Shutemov