[PATCH v5 06/30] arm64: context switch POR_EL0 register

[Will Deacon]

On Thu, Aug 22, 2024 at 04:10:49PM +0100, Joey Gouly wrote:
> POR_EL0 is a register that can be modified by userspace directly,
> so it must be context switched.
> 
> Signed-off-by: Joey Gouly <joey.gouly at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
> ---
>  arch/arm64/include/asm/cpufeature.h |  6 ++++++
>  arch/arm64/include/asm/processor.h  |  1 +
>  arch/arm64/include/asm/sysreg.h     |  3 +++
>  arch/arm64/kernel/process.c         | 28 ++++++++++++++++++++++++++++
>  4 files changed, 38 insertions(+)

[...]

> +static void permission_overlay_switch(struct task_struct *next)
> +{
> +	if (!system_supports_poe())
> +		return;
> +
> +	current->thread.por_el0 = read_sysreg_s(SYS_POR_EL0);
> +	if (current->thread.por_el0 != next->thread.por_el0) {
> +		write_sysreg_s(next->thread.por_el0, SYS_POR_EL0);
> +		/* ISB required for kernel uaccess routines when chaning POR_EL0 */

nit: typo "chaning".

But more substantially, is this just to prevent spurious faults in the
context of a new thread using a stale value for POR_EL0?

Will