linux-kernel - Re: Avoiding unnecessary jump relocations in gas?

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Date:	Wed, 20 May 2015 14:47:10 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"H.J. Lu" <hjl.tools@...il.com>, Borislav Petkov <bp@...en8.de>,
	Jan Beulich <JBeulich@...e.com>,
	Binutils <binutils@...rceware.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Avoiding unnecessary jump relocations in gas?

On Wed, May 20, 2015 at 1:59 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 05/20/2015 01:53 PM, Andy Lutomirski wrote:
>> Egads.  Now I understand what that code is.  I don't like the balign,
>> since this has nothing to do with alignment -- we're creating an array
>> of functions.
>
> Actually it does... we align to the beginning of each slot.  If .balign
> could be something other than a power of 2 that would work, too.

When I see "align", I think that we want to align to a multiple of X
but we don't particularly care which multiple of X.  Here we want a
specific address and any other address would be an error.

>
> I was mostly looking to minimize the amount of gas magic we rely on.
>
>> Can't we make it explicit?
>>
>> #define EARLY_IDT_HANDLER_STRIDE 9
>>
>> ...
>>
>>     .rept NUM_EXCEPTION_VECTORS
>>     . = early_idt_handlers + i * EARLY_IDT_HANDLER_STRIDE
>>     .if (EXCEPTION_ERRCODE_MASK >> i) & 1
>>     ASM_NOP2
>>     .else
>>     pushl $0        # Dummy error code, to make stack frame uniform
>>     .endif
>>     pushl $i        # 20(%esp) Vector number
>>     jmp early_idt_handler
>>     i = i + 1
>>     .endr
>>
>> gas will error out if we try to move . backwards, so this should be safe.
>
> If that works too with all the versions of gas we care about, that would
> be fine (and I do appreciate the explicitness.)  However, .[b]align is
> something that will have been well exercised in every version of gas, so
> I do feel slightly safer with it.

I have no idea how to easily test my approach on really old binutils
versions, but I'm reasonably confident that assigning to . is common.

--Andy
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