Re: [PATCH v2 4/4] arm64: Rework ARM_ERRATUM_1414080 handling

[Mark Rutland]

On Mon, Jul 06, 2020 at 05:38:02PM +0100, Marc Zyngier wrote:
> The current handling of erratum 1414080 has the side effect that
> cntkctl_el1 can get changed for both 32 and 64bit tasks.
> 
> This isn't a problem so far, but if we ever need to mitigate another
> of these errata on the 64bit side, we'd better keep the messing with
> cntkctl_el1 local to 32bit tasks.
> 
> For that, make sure that on entering the kernel from a 32bit tasks,
> userspace access to cntvct gets enabled, and disabled returning to
> userspace, while it never gets changed for 64bit tasks.
> 
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>

The asm looks sound to me. Suggestion below, but either way:

Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>

> ---
>  arch/arm64/kernel/entry.S | 40 +++++++++++++++++++++++----------------
>  1 file changed, 24 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 5304d193c79d..8f51f3273bc7 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -167,6 +167,19 @@ alternative_cb_end
>  	stp	x28, x29, [sp, #16 * 14]
>  
>  	.if	\el == 0
> +	.if	\regsize == 32
> +	// If we come back from a 32bit task on a system affected by
> +	// 1418040, let's reenable userspace access to the virtual counter.
> +#ifdef CONFIG_ARM64_ERRATUM_1418040
> +alternative_if_not ARM64_WORKAROUND_1418040
> +	b	.L__entry_skip_wa_1418040\@
> +alternative_else_nop_endif
> +	mrs	x0, cntkctl_el1
> +	orr	x0, x0, #2	// ARCH_TIMER_USR_VCT_ACCESS_EN
> +	msr	cntkctl_el1, x0
> +.L__entry_skip_wa_1418040\@:
> +#endif

Given this is only 3 instructions, it might be clearer to remove the branch
and label and NOP the whole thing for legibility:

| #ifdef CONFIG_ARM64_ERRATUM_1418040
| alternative_if ARM64_WORKAROUND_1418040
| 	mrs	x0. cntkctl_el1
| 	orr	x0, x0, #2	// ARCH_TIMER_USR_VCT_ACCESS_EN
| 	msr	cntkctl_el1, x0
| alternative_else_nop_endif
| #endif

... and likewise for the clearing case.

Regardless, this looks sound to me, so take your pick. :)

Mark.

> +	.endif
>  	clear_gp_regs
>  	mrs	x21, sp_el0
>  	ldr_this_cpu	tsk, __entry_task, x20
> @@ -318,7 +331,17 @@ alternative_else_nop_endif
>  	ldr	x23, [sp, #S_SP]		// load return stack pointer
>  	msr	sp_el0, x23
>  	tst	x22, #PSR_MODE32_BIT		// native task?
> -	b.eq	3f
> +	b.eq	4f
> +
> +#ifdef CONFIG_ARM64_ERRATUM_1418040
> +alternative_if_not ARM64_WORKAROUND_1418040
> +	b	3f
> +alternative_else_nop_endif
> +	mrs	x1, cntkctl_el1
> +	bic	x1, x1, #2			// ARCH_TIMER_USR_VCT_ACCESS_EN
> +	msr	cntkctl_el1, x1
> +3:
> +#endif
>  
>  #ifdef CONFIG_ARM64_ERRATUM_845719
>  alternative_if ARM64_WORKAROUND_845719
> @@ -330,22 +353,7 @@ alternative_if ARM64_WORKAROUND_845719
>  #endif
>  alternative_else_nop_endif
>  #endif
> -3:
> -#ifdef CONFIG_ARM64_ERRATUM_1418040
> -alternative_if_not ARM64_WORKAROUND_1418040
> -	b	4f
> -alternative_else_nop_endif
> -	/*
> -	 * if (x22.mode32 == cntkctl_el1.el0vcten)
> -	 *     cntkctl_el1.el0vcten = ~cntkctl_el1.el0vcten
> -	 */
> -	mrs	x1, cntkctl_el1
> -	eon	x0, x1, x22, lsr #3
> -	tbz	x0, #1, 4f
> -	eor	x1, x1, #2	// ARCH_TIMER_USR_VCT_ACCESS_EN
> -	msr	cntkctl_el1, x1
>  4:
> -#endif
>  	scs_save tsk, x0
>  
>  	/* No kernel C function calls after this as user keys are set. */
> -- 
> 2.27.0
>