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Message-ID: <1517946292.3677.22.camel@infradead.org>
Subject: Re: [PATCH 2/2] x86/speculation: Simplify
 indirect_branch_prediction_barrier()
From:   David Woodhouse <dwmw2@infradead.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        tim.c.chen@linux.intel.com, pjt@google.com, jikos@kernel.org,
        gregkh@linux-foundation.org, dave.hansen@intel.com,
        riel@redhat.com, luto@amacapital.net,
        torvalds@linux-foundation.org, ak@linux.intel.com,
        keescook@google.com, peterz@infradead.org
In-Reply-To: <20180126200813.cignvfovk2dhlzbh@pd.tnic>
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         <1516970011.30244.223.camel@infradead.org>
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Date:   Tue, 06 Feb 2018 19:44:52 +0000
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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("",
> +			=C2=A0"call __ibp_barrier",
> +			=C2=A0X86_FEATURE_IBPB,
> +			=C2=A0ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
> =C2=A0}

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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