Re: [Xen-devel] [PATCH v6 14/27] x86/percpu: Adapt percpu for PIE support

[Thomas Garnier <thgarnie@xxxxxxxxxxxx>]

On Thu, Jan 31, 2019 at 12:57 PM Christopher Lameter <cl@xxxxxxxxx> wrote:
>
> On Thu, 31 Jan 2019, Thomas Garnier wrote:
>
> > Perpcu uses a clever design where the .percu ELF section has a virtual
> > address of zero and the custom linux relocation code avoid relocating
> > specific symbols. It makes the code simple and easily adaptable with or
> > without SMP support.
>
> We usually talk about this as offsets rather than addressess. The intend
> here is to give every processor its own address that is unique for this
> processor. Operations are always relative to a segment register and the
> whole area can be relocated at will by simply changing the segment
> register.
>
> > This design is incompatible with PIE. While creating a PIE binary, the
> > copmiler tries to make everything relative. The compiler will attempt to
>
> This is very compatible with PIE because it is already relative.

The per-cpu symbols are in a section that is zero based to create
offsets. The compiler doesn't see them as offsets but as relative
symbol and try to relocate them. Given the distance between zero and
the mapped kernel is much larger than the instruction offset range, it
fails to do it.

>
> > generate instructions with the distance between zero and any 64-bit
> > virtual address. It will fail as the relocation range cannot fit within
> > the possible instructions accessing a segment register.
>
> Leave the offsets alone and just change the segment register if you need
> to relocate the area of a specific processor?
>
> > The assembly and PER_CPU macros are changed to use relative references
> > when PIE is enabled.
>
> They already use relative reference. What is the point here?
>
> > --- a/arch/x86/include/asm/percpu.h
> > +++ b/arch/x86/include/asm/percpu.h
> > @@ -5,9 +5,11 @@
> >  #ifdef CONFIG_X86_64
> >  #define __percpu_seg         gs
> >  #define __percpu_mov_op              movq
> > +#define __percpu_rel         (%rip)
>
> The percpu section cannot be IP relative since we need to have separate
> address spaces per cpu.
>

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