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

On Fri, Aug 23, 2024 at 03:45:32PM +0100, Will Deacon wrote:

On Thu, Aug 22, 2024 at 04:10:49PM +0100, Joey Gouly wrote:

...

+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?

Not just prevent faults but enforce the permissions from the new thread's POR_EL0. The kernel may continue with a uaccess routine from here, we can't tell.