linux-kernel - Re: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init

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Message-ID: <CAMzpN2hz4wUFf6Uc6TFovXmT6F-t8kbc9VQd_E=-p+K-TJaK2A@mail.gmail.com>
Date:	Tue, 19 Jan 2016 23:39:43 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...e.de>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init

On Tue, Jan 19, 2016 at 11:02 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 01/19/16 01:22, Borislav Petkov wrote:
>> On Mon, Jan 18, 2016 at 05:33:03PM -0800, H. Peter Anvin wrote:
>>> Why the f do we call a subroutine for what amounts to a single bt or
>>> test instruction?
>>
>> No real reason. You can kick me when you see me next time:
>>
>> 4a90a99c4f80 ("x86: Add a static_cpu_has_safe variant")
>>
>
> So, here is my suggestion:
>
> 1. Just get rid of static_cpu_has_safe() and make static_cpu_has() safe.
>
> 2. Get rid of the non-asm goto variant and just fall back to dynamic if
> asm goto is unavailable.  It doesn't make any sense, really, if it is
> supposed to be safe, and by now the asm goto-capable gcc is in more wide
> use.  (Originally the gcc 3.x fallback to pure dynamic didn't exist,
> either.)
>
> 3. Put the dynamic test in the .init.text section and inline it:
>
>         .section .init.text,"ax"
>         testb %2,%3
>         jnz %[t_yes]
>         jmp %[t_no]
>         .previous
>
>         ... "i" (1 << (bit & 7)),
>             "m" (((const char *)boot_cpu_data->x86_capability)[bit >> 3]) ...

Can't put it in .init.text or else you get:
WARNING: arch/x86/kernel/built-in.o(.text+0x4b9): Section mismatch in
reference from the function __switch_to() to the (unknown reference)
.init.text:(unknown)
The function __switch_to() references
the (unknown reference) __init (unknown).
This is often because __switch_to lacks a __init
annotation or the annotation of (unknown) is wrong.

We want to override that because we know that the reference will be
removed after alternatives run.  That's why I created a new section.

--
Brian Gerst