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SubjectRe: Loongson (and other $ARCHs?) idle VS timer enqueue
On Fri, Apr 21, 2023 at 05:24:36PM +0200, Thomas Gleixner wrote:
> On Fri, Apr 21 2023 at 14:36, Frederic Weisbecker wrote:
> > I'm looking at the __arch_cpu_idle() implementation in Loongarch
> > and I'm wondering about the rollback code. I don't understand well
> > that code but with the help from PeterZ I might have seen something,
> > so tell me if I'm wrong: when an interrupt happens within
> > __arch_cpu_idle(), handle_vint() rolls back the return value to the
> > beginning of __arch_cpu_idle(), so that TIF_NEED_RESCHED is checked
> > again. Is that correct?
> >
> > Because if an interrupt fires while in __arch_cpu_idle(), that IRQ
> > might enqueue a new timer and that new timer needs to be reprogrammed
> > from the main idle loop and just checking TIF_NEED_RESCHED doesn't
> > tell about that information.
>
> The check for TIF_NEED_RESCHED as loop termination condition is simply
> wrong. The architecture is not to supposed to loop in arch_cpu_idle().
>
> That loop is from Linux 0.9 days. Seems MIPS::__r4k_wait() and
> loongarch, which copied that muck are still stuck in the 1990'ies.
>
> It has to return when an interrupt brings it out of the "idle wait"
> instruction.

So I think the below is enough for these two...

diff --git a/arch/loongarch/kernel/genex.S b/arch/loongarch/kernel/genex.S
index 44ff1ff64260..5a102ff80de0 100644
--- a/arch/loongarch/kernel/genex.S
+++ b/arch/loongarch/kernel/genex.S
@@ -40,6 +40,7 @@ SYM_FUNC_START(handle_vint)
ori t0, t0, 0x1f
xori t0, t0, 0x1f
bne t0, t1, 1f
+ addi.d t0, t0, 0x20
LONG_S t0, sp, PT_ERA
1: move a0, sp
move a1, sp
diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
index b6de8e88c1bd..cd6aae441ad9 100644
--- a/arch/mips/kernel/genex.S
+++ b/arch/mips/kernel/genex.S
@@ -140,6 +140,7 @@ LEAF(__r4k_wait)
ori k0, 0x1f /* 32 byte rollback region */
xori k0, 0x1f
bne k0, k1, \handler
+ addiu k0, 0x20
MTC0 k0, CP0_EPC
.set pop
.endm
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