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ref: 4091eb6fcd02ae5162423788f7da23544a28a3a6
parent: 8bcc99861e091e22f0950be771fdb1f1c01a35a0
author: cinap_lenrek <cinap_lenrek@felloff.net>
date: Sat Apr 12 16:06:00 EDT 2025

kernel/arm64: fix procsetup() TPIDR_EL0 potentially not getting reset to 0 on exec

We have to be carefull with the order here.

Imagine we get preepted immediately after we set p->tpidr = 0,
procsave() would override up->tpidr again with the current
register value. once we get switched back in, we then
execute syswr(TPIDR_EL0, p->tpidr), failing to reset the
register to 0.

So the correct order is to first set the register, and
then we can (optionally) set p->tpidr = 0 (doesnt matter).

--- a/sys/src/9/arm64/trap.c

+++ b/sys/src/9/arm64/trap.c

@@ -505,8 +505,8 @@

 procsetup(Proc *p)

 {

 	fpuprocsetup(p);

+	syswr(TPIDR_EL0, 0);

 	p->tpidr = 0;

-	syswr(TPIDR_EL0, p->tpidr);

 }

 void

-- 

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