Re: [PATCH] x86/retpoline: Fill return stack buffer on vmexit

[Tim Chen]

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