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Subject: Re: [PATCH] x86/retpoline: Fill return stack buffer on vmexit
To: David Woodhouse <dwmw@amazon.co.uk>,
        Andi Kleen <ak@linux.intel.com>
Cc: Paul Turner <pjt@google.com>, LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>, tglx@linutronix.de,
        Kees Cook <keescook@google.com>,
        Rik van Riel <riel@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Jiri Kosina <jikos@kernel.org>, gnomes@lxorguk.ukuu.org.uk,
        x86@kernel.org, bp@alien8.de, rga@amazon.de
References: <1515624682-3556-1-git-send-email-dwmw@amazon.co.uk>
From: Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <5371f9a0-f221-af1f-641f-05e4d0991847@linux.intel.com>
Date: Wed, 10 Jan 2018 15:47:35 -0800
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On 01/10/2018 02:51 PM, David Woodhouse wrote:

> + */
> +#define __FILL_RETURN_BUFFER(reg, sp, uniq)	\
> +	mov	$(NUM_BRANCHES_TO_FILL/2), reg;	\
> +	.align	16;				\
> +.Ldo_call1_ ## uniq:				\
> +	call	.Ldo_call2_ ## uniq;		\
> +.Ltrap1_ ## uniq:				\
> +	pause;					\
> +	jmp	.Ltrap1_ ## uniq;		\
> +	.align	16;				\
> +.Ldo_call2_ ## uniq:				\
> +	call	.Ldo_loop_ ## uniq;		\
> +.Ltrap2_ ## uniq:				\
> +	pause;					\
> +	jmp	.Ltrap2_ ## uniq;		\
> +	.align	16;				\
> +.Ldo_loop_ ## uniq:				\
> +	dec	reg;				\
> +	jnz	.Ldo_call1_ ## uniq;		\
> +	add	$(BITS_PER_LONG/8)*NUM_BRANCHES_TO_FILL, sp;
> +
>  #ifdef __ASSEMBLY__
>  

> +
> +	asm volatile (ALTERNATIVE("",
> +				  __stringify(__FILL_RETURN_BUFFER(%0, %1, _%=)),
> +				  X86_FEATURE_RETPOLINE)

We'll be patching in a fairly long set of instructions here.  Maybe put
the ALTERNATIVE in the assembly and use a jmp skip_\@ for the ALTERNATIVE.

Thanks.

Tim