[PATCH v2 1/2] arm64: clear_page() shouldn't use DC ZVA when DCZID_EL0.DZP == 1

[Robin Murphy]

On 2021-11-08 07:11, Reiji Watanabe wrote:
> Currently, clear_page() uses DC ZVA instruction unconditionally.  But it
> should make sure that DCZID_EL0.DZP, which indicates whether or not use
> of DC ZVA instruction is prohibited, is zero when using the instruction.
> Use STP as memset does instead when DCZID_EL0.DZP == 1.

Reviewed-by: Robin Murphy <robin.murphy at arm.com>

FWIW I did eventually figure out the "pre-bias" trick from v1 thanks to 
Mark's nod toward the original context, but a quick survey of various 
optimisation guides implied that the explicit add should generally be 
preferred over post-index writeback anyway, so I think we're all good here.

Cheers,
Robin.

> Fixes: f27bb139c387 ("arm64: Miscellaneous library functions")
> Signed-off-by: Reiji Watanabe <reijiw at google.com>
> ---
>   arch/arm64/lib/clear_page.S | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/arch/arm64/lib/clear_page.S b/arch/arm64/lib/clear_page.S
> index b84b179edba3..ea3a6c927fe8 100644
> --- a/arch/arm64/lib/clear_page.S
> +++ b/arch/arm64/lib/clear_page.S
> @@ -16,6 +16,7 @@
>    */
>   SYM_FUNC_START_PI(clear_page)
>   	mrs	x1, dczid_el0
> +	tbnz	x1, #4, 2f	/* Branch if DC ZVA is prohibited */
>   	and	w1, w1, #0xf
>   	mov	x2, #4
>   	lsl	x1, x2, x1
> @@ -25,5 +26,14 @@ SYM_FUNC_START_PI(clear_page)
>   	tst	x0, #(PAGE_SIZE - 1)
>   	b.ne	1b
>   	ret
> +
> +2:	stp	xzr, xzr, [x0]
> +	stp	xzr, xzr, [x0, #16]
> +	stp	xzr, xzr, [x0, #32]
> +	stp	xzr, xzr, [x0, #48]
> +	add	x0, x0, #64
> +	tst	x0, #(PAGE_SIZE - 1)
> +	b.ne	2b
> +	ret
>   SYM_FUNC_END_PI(clear_page)
>   EXPORT_SYMBOL(clear_page)
>