On Sat, Mar 15, 2025 at 08:40:02PM +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. can you give a commit ID, when this change got introduced ? > 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. > > CONFIG_CPU_MICROMIPS has been removed along with the nop instructions. > There, NOPs are 2 byte in size, so change the code with 3 _ssnop which are > always 4 byte and remove the ifdef. Added ehb to make sure the hazard > is always cleared. > > Signed-off-by: Marco Crivellari <marco.crivellari@xxxxxxxx> > --- > arch/mips/kernel/genex.S | 42 ++++++++++++++++++++++------------------ > arch/mips/kernel/idle.c | 1 - > 2 files changed, 23 insertions(+), 20 deletions(-) > > diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S > index a572ce36a24f..4e012421d00f 100644 > --- a/arch/mips/kernel/genex.S > +++ b/arch/mips/kernel/genex.S > @@ -104,27 +104,30 @@ 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 > + /* Start of idle interrupt region. */ > + MFC0 t0, CP0_STATUS > + /* Enable interrupt. */ > + ori t0, 0x1f > + xori t0, 0x1e > + MTC0 t0, CP0_STATUS > + _ssnop > + _ssnop > + _ssnop > + _ehb > .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) */ > + /* End of idle interrupt region. */ > 1: please give this label a name for example __r4k_wait_exit and do a runtime check that it really has 36 bytes offset to __r4k_wait > jr ra > nop > @@ -136,9 +139,10 @@ 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 this is part of a macro, so I don't think using a commonly used label name is a safe thing, that's why I want a named label here. > + /* 36 byte idle interrupt region. */ > + ori k0, 0x1f > + PTR_ADDIU k0, 5 > bne k0, k1, \handler > MTC0 k0, CP0_EPC > .set pop -- Crap can work. Given enough thrust pigs will fly, but it's not necessarily a good idea. [ RFC1925, 2.3 ]
