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Message-ID: <1516970011.30244.223.camel@infradead.org>
Subject: Re: [PATCH 3/4] x86/retpoline: Simplify vmexit_fill_RSB()
From:   David Woodhouse <dwmw2@infradead.org>
To:     Borislav Petkov <bp@alien8.de>, X86 ML <x86@kernel.org>
Cc:     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: <20180126121139.31959-4-bp@alien8.de>
References: <20180126121139.31959-1-bp@alien8.de>
         <20180126121139.31959-4-bp@alien8.de>
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Date:   Fri, 26 Jan 2018 12:33:31 +0000
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On Fri, 2018-01-26 at 13:11 +0100, Borislav Petkov wrote:
>=20
> +ENTRY(__fill_rsb_clobber_ax)
> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0___FILL_RETURN_BUFFER %_ASM_AX=
, RSB_CLEAR_LOOPS, %_ASM_SP
> +END(__fill_rsb_clobber_ax)
> +EXPORT_SYMBOL_GPL(__fill_rsb_clobber_ax)

You still have clear vs. fill confusion there.

How about making it take the loop count in %eax? That would allow us to
drop the ___FILL_RETURN_BUFFER macro entirely.

Or does that make us depend on your other fixes to accept jumps in
places other than the first instruction of altinstr?=C2=A0

Even if you give us separate __clear_rsb_clobber_ax vs.
__fill_rsb_clobber_ax functions, we could still kill the macro in
nospec-branch.h and use a .macro in retpoline.S for the actual
implementation, couldn't we?

> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -971,4 +971,9 @@ bool xen_set_default_idle(void);
> =C2=A0
> =C2=A0void stop_this_cpu(void *dummy);
> =C2=A0void df_debug(struct pt_regs *regs, long error_code);
> +
> +#ifdef CONFIG_RETPOLINE
> +asmlinkage void __fill_rsb_clobber_ax(void);
> +#endif
> +
> =C2=A0#endif /* _ASM_X86_PROCESSOR_H */

Doesn't that live in asm-prototypes.h? Don't make it visible to any C
code; it *isn't* a C function.
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