Re: [Xen-devel] [PATCH 3/4] xen/link: Fold .data.read_mostly into .data

[Julien Grall <julien.grall@arm.com>]

On 6/19/19 9:11 PM, Andrew Cooper wrote:
> .data.read_mostly only needs separating from adjacent data by one cache line
> to be effective, and placing it adjacent to .data.page_aligned fulfills this
> requirement.
> 
> For ARM, .ex_table appears to be a vestigial remnant.  Nothing in the
> resulting build ever inspects or acts on the contents of the table.  The arm32
> build does however have some assembly routines which fill .ex_table.
> 
> Drop all of ARM's .ex_table infrastructure, to reduce the size of the compiled
> binary, and avoid giving the illusion of exception handling working.

I am not in favor of this change. assembler.h is meant to be a verbatim 
copy of the Linux counterpart.

[...]

> diff --git a/xen/arch/arm/xen.lds.S b/xen/arch/arm/xen.lds.S
> index 2b44e5d..3dc5117 100644
> --- a/xen/arch/arm/xen.lds.S
> +++ b/xen/arch/arm/xen.lds.S
> @@ -94,27 +94,13 @@ SECTIONS
>     _erodata = .;                /* End of read-only data */
>   
>     .data : {                    /* Data */
> +       *(.data.read_mostly)

Before, .data.read_mostly was SMP_CACHE_BYTES aligned. Now, it seems 
there are no alignment.

This may end up to have _erodata and .data.read_mostly to be part of the 
same page. As Arm enforce read-only, you may crash on access to 
.data.read_mostly.

So I think you have
. = ALIGN(PAGE_SIZE)
*(.data.read_mostly)
.align(SMP_CACHE_BYTES).

Cheers,

-- 
Julien Grall

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