linux-kernel - Re: [PATCH 08/24] x86,sme: Annotate indirect call

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Date:   Mon, 29 Jan 2018 18:49:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 08/24] x86,sme: Annotate indirect call

On Fri, Jan 26, 2018 at 10:37:30AM +0000, David Woodhouse wrote:
> On Tue, 2018-01-23 at 16:25 +0100, Peter Zijlstra wrote:
> > This is boot code, we run this _way_ before userspace comes along to
> > poison our branch predictor.
> 
> Hm, objtool knows about sections, doesn't it? Why it is whining about
> indirect jumps in inittext anyway?
> 
> In fact, why are we even *doing* retpolines in inittext? Not that we
> are; since we flipped the ALTERNATIVE logic around, at that point we
> still have the 'oldinstr' which is a bare jmp anyway. We might as well
> do this:
> 
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -37,10 +37,15 @@
>   * as gcc otherwise puts the data into the bss section and not into the init
>   * section.
>   */
> +#if defined(RETPOLINE) && !defined(MODULE)
> +#define __noretpoline __attribute__((indirect_branch("keep")))
> +#else
> +#define __noretpoline
> +#endif
>  
>  /* These are for everybody (although not all archs will actually
>     discard it in modules) */
> -#define __init         __section(.init.text) __cold __inittrace __latent_entropy
> +#define __init         __section(.init.text) __cold __inittrace __latent_entropy __noretpoline

We run module __init text concurrently with userspace.