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Re: [Xen-devel] Hypervisor compilation for arm without debug=y

On Mon, 2013-08-19 at 14:44 +0300, Andrii Anisov wrote:
> Hello, 
> I tried to get hypervisor built without debug=y option.
> Unfortunately built binary doesn't boot.
> I've localized a problem and get a workaround for it:


> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index d1290cd..aa8e057 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -332,7 +332,7 @@ void __cpuinit setup_virt_paging(void)
>  /* Boot-time pagetable setup.
>   * Changes here may need matching changes in head.S */
> -void __init setup_pagetables(unsigned long boot_phys_offset, paddr_t 
> xen_paddr)
> +void __attribute__((optimize("-fno-omit-frame-pointer"))) __init 
> setup_pagetables(unsigned long boot_phys_offset, paddr_t xen_paddr)
>  {
>      unsigned long dest_va;
>      lpae_t pte, *p;
> Is it a known issue?

I suspect none of us has ever really used a non-debug build!

> Is there a better solution for it?

So the confusing double negative -fno-omit-frame-pointer is causing the
frame pointer to be omitted for this function?

I wonder why the frame pointer is a problem here. This function is a bit
sensitive since it relocates Xen in physical RAM but it doesn't change
the virtual address space so I would expect fp, sp etc to remain valid.

The danger is if something is written to the stack between the initial
copy and the switch over, but I'd have thought that the frame pointer
would be written to the stack near the start and popped at the end and
mostly not touched in the middle.

I guess what I'm trying to say is I'm not confident that adding that
flag has "fixed" the issue, rather than just causing the compiler to
behave a bit differently and avoid it.

Are you able to post any details about the boot failure? Is it a crash
or something more subtle? Are you able to localise it to a particular
instruction or section of code?

> I work with 4.3 release, build with gcc version 4.6.3 (Ubuntu/Linaro
> 4.6.3-1ubuntu5) 

For ARM stuff you are probably better off tracking mainline (unstable)
xen. Depending on what you are doing of course.


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