linux-kernel - Re: [PATCH 2/2] x86/speculation: Simplify indirect_branch_prediction

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Date:   Tue, 06 Feb 2018 19:44:52 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        tim.c.chen@...ux.intel.com, pjt@...gle.com, jikos@...nel.org,
        gregkh@...ux-foundation.org, dave.hansen@...el.com,
        riel@...hat.com, luto@...capital.net,
        torvalds@...ux-foundation.org, ak@...ux.intel.com,
        keescook@...gle.com, peterz@...radead.org
Subject: Re: [PATCH 2/2] x86/speculation: Simplify
 indirect_branch_prediction_barrier()

On Fri, 2018-01-26 at 21:08 +0100, Borislav Petkov wrote:
> Make it all a function which does the WRMSR instead of having a hairy
> inline asm.

...

> +	alternative_input("",
> +			 "call __ibp_barrier",
> +			 X86_FEATURE_IBPB,
> +			 ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
>  }

Dammit. I know the best time to comment is *before* I add my own sign-
off to it and before Linus has merged it but... I think this is broken.

If you're calling a C function then you have to mark *all* the call-
clobbered registers as, well, clobbered.

If you really really really want to *call* something out of line, then
it would need to be implemented in asm.
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