Re: [PATCH v2 1/1] MIPS: Fix idle VS timer enqueue

[Thomas Bogendoerfer]

On Tue, Feb 18, 2025 at 10:02:03AM +0100, Marco Crivellari wrote:
> MIPS re-enables interrupts on its idle routine and performs
> a TIF_NEED_RESCHED check afterwards before putting the CPU to sleep.
> 
> The IRQs firing between the check and the 'wait' instruction may set the
> TIF_NEED_RESCHED flag. In order to deal with this possible race, IRQs
> interrupting __r4k_wait() rollback their return address to the
> beginning of __r4k_wait() so that TIF_NEED_RESCHED is checked
> again before going back to sleep.
> 
> However idle IRQs can also queue timers that may require a tick
> reprogramming through a new generic idle loop iteration but those timers
> would go unnoticed here because __r4k_wait() only checks
> TIF_NEED_RESCHED. It doesn't check for pending timers.
> 
> Fix this with fast-forwarding idle IRQs return address to the end of the
> idle routine instead of the beginning, so that the generic idle loop
> handles both TIF_NEED_RESCHED and pending timers.
> 
> Signed-off-by: Marco Crivellari <marco.crivellari@xxxxxxxx>
> ---
>  arch/mips/kernel/genex.S | 39 +++++++++++++++++++++------------------
>  arch/mips/kernel/idle.c  |  1 -
>  2 files changed, 21 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
> index a572ce36a24f..9747b216648f 100644
> --- a/arch/mips/kernel/genex.S
> +++ b/arch/mips/kernel/genex.S
> @@ -104,25 +104,27 @@ handle_vcei:
>  
>  	__FINIT
>  
> -	.align	5	/* 32 byte rollback region */
> +	.align	5
>  LEAF(__r4k_wait)
>  	.set	push
>  	.set	noreorder
> -	/* start of rollback region */
> -	LONG_L	t0, TI_FLAGS($28)
> -	nop
> -	andi	t0, _TIF_NEED_RESCHED
> -	bnez	t0, 1f
> -	 nop
> -	nop
> -	nop
> -#ifdef CONFIG_CPU_MICROMIPS
> -	nop
> -	nop
> -	nop
> -	nop
> -#endif

My quick search didnn't find the reason for the extra NOPs on MICROMIPS, but
they are here for a purpose. I might still need them...

> +	/* start of idle interrupt region */
> +	MFC0	t0, CP0_STATUS
> +	/* Enable Interrput */
> +	ori 	t0, 0x1f
> +	xori	t0, 0x1e
> +	MTC0	t0, CP0_STATUS
> +	_ssnop
> +	_ssnop
> +	_ssnop

instead of handcoded hazard nops, use __irq_enable_hazard for that

>  	.set	MIPS_ISA_ARCH_LEVEL_RAW
> +	/*
> +	 * If an interrupt lands here, between enabling interrupts above and
> +	 * going idle on the next instruction, we must *NOT* go idle since the
> +	 * interrupt could have set TIF_NEED_RESCHED or caused a timer to need
> +	 * resched. Fall through -- see rollback_handler below -- and have
> +	 * the idle loop take care of things.
> +	 */
>  	wait
>  	/* end of rollback region (the region size must be power of two) */
>  1:
> @@ -136,9 +138,10 @@ LEAF(__r4k_wait)
>  	.set	push
>  	.set	noat
>  	MFC0	k0, CP0_EPC
> -	PTR_LA	k1, __r4k_wait
> -	ori	k0, 0x1f	/* 32 byte rollback region */
> -	xori	k0, 0x1f
> +	PTR_LA	k1, 1b
> +	/* 32 byte idle interrupt region */
> +	ori 	k0, 0x1f
> +	daddiu	k0, 1

/local/tbogendoerfer/korg/linux/arch/mips/kernel/genex.S: Assembler messages:
/local/tbogendoerfer/korg/linux/arch/mips/kernel/genex.S:151: Error: opcode not supported on this processor: mips32r2 (mips32r2) `daddiu $26,1'
/local/tbogendoerfer/korg/linux/arch/mips/kernel/genex.S:271: Error: opcode not supported on this processor: mips32r2 (mips32r2) `daddiu $26,1'

looks like you haven't compiled this code for 32bit. Use PTR_ADDIU, which
will use the correct instuction for 32 and 64bit.

But I doubt this works, because the wait instruction is not aligned to
a 32 byte boundary, but the code assuemes this, IMHO.

Thomas.

-- 
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