From: "H. Peter Anvin" <hpa@zytor.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: "H.J. Lu" <hjl.tools@gmail.com>, Borislav Petkov <bp@alien8.de>,
Jan Beulich <JBeulich@suse.com>,
Binutils <binutils@sourceware.org>,
"linux-kernel@vger.kernel.org"
<linux-kernel@vger.kernel.org>
Subject: Re: Avoiding unnecessary jump relocations in gas?
Date: Wed, 20 May 2015 22:09:00 -0000 [thread overview]
Message-ID: <1B20BA6B-BF17-4A3E-AE65-49828B708B5C@zytor.com> (raw)
In-Reply-To: <CALCETrU021B2kMouswTuojwXMYGi3CX-gEqhk2+prYkxhi4ygQ@mail.gmail.com>
Also, obviously, let's get rid of the now unnecessary ASM_NOP2.
On May 20, 2015 2:47:10 PM PDT, Andy Lutomirski <luto@amacapital.net> wrote:
>On Wed, May 20, 2015 at 1:59 PM, H. Peter Anvin <hpa@zytor.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
--
Sent from my mobile phone. Please pardon brevity and lack of formatting.
next prev parent reply other threads:[~2015-05-20 22:09 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-07 6:02 Andy Lutomirski
2015-05-07 11:52 ` Jan Beulich
2015-05-07 16:21 ` H.J. Lu
2015-05-08 3:23 ` Andy Lutomirski
2015-05-08 12:09 ` H.J. Lu
2015-05-08 20:16 ` H.J. Lu
2015-05-18 19:37 ` Andy Lutomirski
2015-05-18 20:02 ` H.J. Lu
2015-05-18 20:06 ` H. Peter Anvin
2015-05-18 20:25 ` H.J. Lu
2015-05-18 20:28 ` H. Peter Anvin
2015-05-18 20:34 ` H. Peter Anvin
2015-05-20 20:54 ` Andy Lutomirski
2015-05-20 21:00 ` H. Peter Anvin
2015-05-20 21:47 ` Andy Lutomirski
2015-05-20 22:00 ` H. Peter Anvin
2015-05-20 22:09 ` H. Peter Anvin [this message]
2015-05-20 22:18 ` Andy Lutomirski
2015-05-20 22:22 ` H. Peter Anvin
2015-05-18 20:07 ` H. Peter Anvin
2015-05-18 20:11 ` Borislav Petkov
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