Re: [PATCH ARM64 v4.4 V3 02/44] arm64: Implement array_index_mask_nospec()

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Aug 29, 2019 at 05:03:47PM +0530, Viresh Kumar wrote:
> From: Robin Murphy <robin.murphy@arm.com>
> 
> commit 022620eed3d0bc4bf2027326f599f5ad71c2ea3f upstream.
> 
> Provide an optimised, assembly implementation of array_index_mask_nospec()
> for arm64 so that the compiler is not in a position to transform the code
> in ways which affect its ability to inhibit speculation (e.g. by introducing
> conditional branches).
> 
> This is similar to the sequence used by x86, modulo architectural differences
> in the carry/borrow flags.
> 
> Reviewed-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

Reviewed-by: Mark Rutland <mark.rutland@arm.com> [v4.4 backport]

Mark.

> ---
>  arch/arm64/include/asm/barrier.h | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 574486634c62..7c25e3e11b6d 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -37,6 +37,27 @@
>  #define dma_rmb()	dmb(oshld)
>  #define dma_wmb()	dmb(oshst)
>  
> +/*
> + * Generate a mask for array_index__nospec() that is ~0UL when 0 <= idx < sz
> + * and 0 otherwise.
> + */
> +#define array_index_mask_nospec array_index_mask_nospec
> +static inline unsigned long array_index_mask_nospec(unsigned long idx,
> +						    unsigned long sz)
> +{
> +	unsigned long mask;
> +
> +	asm volatile(
> +	"	cmp	%1, %2\n"
> +	"	sbc	%0, xzr, xzr\n"
> +	: "=r" (mask)
> +	: "r" (idx), "Ir" (sz)
> +	: "cc");
> +
> +	csdb();
> +	return mask;
> +}
> +
>  #define smp_mb()	dmb(ish)
>  #define smp_rmb()	dmb(ishld)
>  #define smp_wmb()	dmb(ishst)
> -- 
> 2.21.0.rc0.269.g1a574e7a288b
>