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Date: Sun, 21 Jan 2018 20:53:03 +0100
From: Borislav Petkov <bp@alien8.de>
To: David Woodhouse <dwmw2@infradead.org>
Cc: KarimAllah Ahmed <karahmed@amazon.com>, arjan@linux.intel.com,
	tglx@linutronix.de, karahmed@amazon.de, x86@kernel.org,
	linux-kernel@vger.kernel.org, tim.c.chen@linux.intel.com,
	peterz@infradead.org, pbonzini@redhat.com, ak@linux.intel.com,
	torvalds@linux-foundation.org, gregkh@linux-foundation.org
Subject: Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB
Message-ID: <20180121195303.afedou2l4ar37e4r@pd.tnic>
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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.

Good mailing practices for 400: avoid top-posting and trim the reply.