Re: [PATCH v4 3/5] build: Alloc space for sched list in the link file

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 08/01/16 21:22, Jonathan Creekmore wrote:
> Creates a section to contain scheduler entry pointers that are gathered
> together into an array. This will allow, in a follow-on patch, scheduler
> entries to be automatically gathered together into the array for
> automatic parsing.
>
> CC: Ian Campbell <ian.campbell@citrix.com>
> CC: Stefano Stabellini <stefano.stabellini@citrix.com>
> CC: Keir Fraser <keir@xen.org>
> CC: Jan Beulich <jbeulich@suse.com>
> CC: Andrew Cooper <andrew.cooper3@citrix.com>
> Signed-off-by: Jonathan Creekmore <jonathan.creekmore@gmail.com>
> Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
>
> ---
> Changed since v3:
>   * Add defensive check for schedulers in the linker
>
> Changed since v1:
>   * rename the __start and __end symbols to better match
>     the rest of the file
> ---
>  xen/arch/arm/xen.lds.S | 5 +++++
>  xen/arch/x86/xen.lds.S | 5 +++++
>  2 files changed, 10 insertions(+)
>
> diff --git a/xen/arch/arm/xen.lds.S b/xen/arch/arm/xen.lds.S
> index 0488f37..2492def 100644
> --- a/xen/arch/arm/xen.lds.S
> +++ b/xen/arch/arm/xen.lds.S
> @@ -57,6 +57,10 @@ SECTIONS
>         . = ALIGN(PAGE_SIZE);
>         *(.data.page_aligned)
>         *(.data)
> +       . = ALIGN(8);
> +       __start_schedulers_array = .;
> +       *(.data.schedulers)
> +       __end_schedulers_array = .;
>         *(.data.rel)
>         *(.data.rel.*)
>         CONSTRUCTORS
> @@ -193,3 +197,4 @@ SECTIONS
>   * code running on the boot time identity map cannot cross a section boundary.
>   */
>  ASSERT( _end_boot - start <= PAGE_SIZE, "Boot code is larger than 4K")
> +ASSERT((__end_schedulers_array - __start_schedulers_array) > 0, "no schedulers compiled in")
> diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
> index e18e08f..63f89af 100644
> --- a/xen/arch/x86/xen.lds.S
> +++ b/xen/arch/x86/xen.lds.S
> @@ -80,6 +80,10 @@ SECTIONS
>         __stop___pre_ex_table = .;
>  
>         *(.data.read_mostly)
> +       . = ALIGN(8);
> +       __start_schedulers_array = .;
> +       *(.data.schedulers)
> +       __end_schedulers_array = .;
>         *(.data.rel.ro)
>         *(.data.rel.ro.*)
>    } :text
> @@ -226,3 +230,4 @@ ASSERT(kexec_reloc_size - kexec_reloc <= PAGE_SIZE, "kexec_reloc is too large")
>  #endif
>  
>  ASSERT((cpu0_stack & (STACK_SIZE - 1)) == 0, "cpu0_stack misaligned")
> +ASSERT((__end_schedulers_array - __start_schedulers_array) > 0, "no schedulers compiled in")

This patch won't build on its own, as the ASSERT() will fire. 
Therefore, it breaks bisectability.

In this patch (or the previous one), you need to move the schedulers
list into __section(".data.schedulers"), and undo the movement in patch
4 when populating .data.schedulers properly.

Alternatively, you could just merge patches 3 and 4.  I don't think that
would reduce the clarity of what you were doing.

~Andrew