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Date: Sat, 6 Jan 2018 10:35:41 -0800
From: Eric Biggers <ebiggers3@gmail.com>
To: David Woodhouse <dwmw@amazon.co.uk>
Cc: Andi Kleen <ak@linux.intel.com>, Paul Turner <pjt@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Andy Lutomirski <luto@amacapital.net>,
        Jiri Kosina <jikos@kernel.org>, gnomes@lxorguk.ukuu.org.uk
Subject: Re: [PATCH v5 02/12] x86/retpoline: Add initial retpoline support
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On Sat, Jan 06, 2018 at 11:49:24AM +0000, David Woodhouse wrote:
> +/*
> + * NOSPEC_JMP and NOSPEC_CALL macros can be used instead of a simple
> + * indirect jmp/call which may be susceptible to the Spectre variant 2
> + * attack.
> + */

Can be, or must be?

> +.macro NOSPEC_JMP reg:req
> +#ifdef CONFIG_RETPOLINE
> +	ALTERNATIVE_2 __stringify(jmp *\reg),				\
> +		__stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE,	\
> +		__stringify(lfence; jmp *\reg), X86_FEATURE_RETPOLINE_AMD
> +#else
> +	jmp	*\reg
> +#endif
> +.endm
> +
> +.macro NOSPEC_CALL reg:req
> +#ifdef CONFIG_RETPOLINE
> +	ALTERNATIVE_2 __stringify(call *\reg),				\
> +		__stringify(RETPOLINE_CALL \reg), X86_FEATURE_RETPOLINE,\
> +		__stringify(lfence; call *\reg), X86_FEATURE_RETPOLINE_AMD
> +#else
> +	call	*\reg
> +#endif
> +.endm

Would it make any sense to name these INDIRECT_JMP and INDIRECT_CALL instead?
NOSPEC_ seems to describe how it needs to be implemented on some CPUs, as
opposed to what the user wants to do (make an indirect jump or call).

Eric