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Date: Mon, 29 Jan 2018 18:49:52 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Josh Poimboeuf <jpoimboe@redhat.com>, linux-kernel@vger.kernel.org,
	Dave Hansen <dave.hansen@intel.com>,
	Ashok Raj <ashok.raj@intel.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andi Kleen <ak@linux.intel.com>,
	Arjan Van De Ven <arjan.van.de.ven@intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Jun Nakajima <jun.nakajima@intel.com>,
	Asit Mallick <asit.k.mallick@intel.com>,
	Jason Baron <jbaron@akamai.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Borislav Petkov <bp@suse.de>
Subject: Re: [PATCH 08/24] x86,sme: Annotate indirect call
Message-ID: <20180129174952.GI2228@hirez.programming.kicks-ass.net>
References: <20180123152539.374360046@infradead.org>
 <20180123152638.453859448@infradead.org>
 <1516963050.30244.188.camel@infradead.org>
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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.