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Re: [PATCH] x86/boot: re-order .init.data contributions
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 3 Apr 2025 14:06:56 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 03 Apr 2025 12:07:03 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 03.04.2025 14:02, Andrew Cooper wrote:
> On 01/04/2025 6:56 am, Jan Beulich wrote:
>> Putting a few bytes ahead of page tables isn't very efficient; there's
>> a gap almost worth a full page. To avoid re-ordering of items in the
>> source file, simply put the few small items in sub-section 1, for them
>> to end up after the page tables, followed (in the final binary) by non-
>> page-aligned items from other CUs.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> --- a/xen/arch/x86/boot/head.S
>> +++ b/xen/arch/x86/boot/head.S
>> @@ -136,6 +136,7 @@ multiboot2_header:
>> .Lno_nx_msg: .asciz "ERR: Not an NX-capable CPU!"
>>
>> .section .init.data, "aw", @progbits
>> + .subsection 1 /* Put data here after the page tables (in x86_64.S).
>> */
>> .align 4
>>
>> .word 0
>
> Just as a note, this depends on the toolchain baseline bump, so can't be
> backported.
>
> Clang-11 does now like subsection, so we can remove the !__clang__
> specialisation for UNLIKELY_{START,END}_SECTION.
Funny you should mention that: I did check before even making the change, and
in assembly code we used .subsection already before. We just needed to be
careful about inline assembly. See UNLIKELY_START() vs the two forms of
UNLIKELY_START_SECTION.
Irrespective of that: I had no intention to backport this.
Jan
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