Re: [RFC] arm64: Ensure proper addressing for ldnp/stnp

[Robin Murphy]

On 19/09/16 18:36, Brent DeGraaf wrote:
> According to section 6.3.8 of the ARM Programmer's Guide, non-temporal
> loads and stores do not verify that address dependency is met between a
> load of an address to a register and a subsequent non-temporal load or
> store using that address on the executing PE. Therefore, context switch
> code and subroutine calls that use non-temporally accessed addresses as
> parameters that might depend on a load of an address into an argument
> register must ensure that ordering requirements are met by introducing
> a barrier prior to the successive non-temporal access.  Add appropriate
> barriers whereever this specific situation comes into play.
> 
> Signed-off-by: Brent DeGraaf <bdegraaf@xxxxxxxxxxxxxx>
> ---
>  arch/arm64/kernel/entry.S  | 1 +
>  arch/arm64/lib/copy_page.S | 2 ++
>  2 files changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 441420c..982c4d3 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -679,6 +679,7 @@ ENTRY(cpu_switch_to)
>  	ldp	x27, x28, [x8], #16
>  	ldp	x29, x9, [x8], #16
>  	ldr	lr, [x8]
> +	dmb	nshld	// Existence of instructions with loose load-use dependencies (e.g. ldnp/stnp) make this barrier necessary
>  	mov	sp, x9
>  	and	x9, x9, #~(THREAD_SIZE - 1)
>  	msr	sp_el0, x9
> diff --git a/arch/arm64/lib/copy_page.S b/arch/arm64/lib/copy_page.S
> index 4c1e700..21c6892 100644
> --- a/arch/arm64/lib/copy_page.S
> +++ b/arch/arm64/lib/copy_page.S
> @@ -47,6 +47,8 @@ alternative_endif
>  	ldp	x14, x15, [x1, #96]
>  	ldp	x16, x17, [x1, #112]
>  
> +	dmb	nshld // In case x0 (for stnp) is dependent on a load

The ARMv8 ARM (B2.7.2 in issue j) says that when an address dependency
exists between a load and a subsequent LDNP, *other* observers may
observe those accesses in any order. How's that related to an STNP on
the same CPU?

Robin.

> +
>  	mov	x18, #(PAGE_SIZE - 128)
>  	add	x1, x1, #128
>  1:
>