linux-kernel - Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB

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Message-ID: <20180121195303.afedou2l4ar37e4r@pd.tnic>
Date:   Sun, 21 Jan 2018 20:53:03 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     KarimAllah Ahmed <karahmed@...zon.com>, arjan@...ux.intel.com,
        tglx@...utronix.de, karahmed@...zon.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, tim.c.chen@...ux.intel.com,
        peterz@...radead.org, pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org
Subject: Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB

On Sun, Jan 21, 2018 at 07:31:00PM +0000, David Woodhouse wrote:
> Congratulations, you've just turned a potential GCC missed optimisation
> into a kernel bug. We don't *care* that it's unlikely that GCC will
> miss that optimisation. The point is that it doesn't *have* to do it,
> and we don't *check*.

Err, I guess I'm missing something:

gcc doesn't add any code here - we do. We turn the JMP into a NOP and
the three instructions of the wrmsr are from an asm volatile().

So please elaborate.

Thx.

-- 
Regards/Gruss,
    Boris.

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