Re: [PATCH 1/2] arm64/entry: Fix involuntary preemption exception masking

[Jinjie Ruan]

On 2026/3/25 23:46, Thomas Gleixner wrote:
> On Wed, Mar 25 2026 at 11:03, Mark Rutland wrote:
>> On Sun, Mar 22, 2026 at 12:25:06AM +0100, Thomas Gleixner wrote:
>>> The current sequence on entry is:
>>>
>>>   // interrupts are disabled by interrupt/exception entry
>>>   enter_from_kernel_mode()
>>>      irqentry_enter(regs);
>>>      mte_check_tfsr_entry();
>>>      mte_disable_tco_entry();
>>>      daif_inherit(regs);
>>>      // interrupts are still disabled
>>
>> That last comment isn't quite right: we CAN and WILL enable interrupts
>> in local_daif_inherit(), if and only if they were enabled in the context
>> the exception was taken from.
> 
> Ok.
> 
>> As mentioned above, when handling an interrupt (rather than a
>> synchronous exception), we don't use local_daif_inherit(), and instead
>> use a different DAIF function to unmask everything except interrupts.
>>
>>> which then becomes:
>>>
>>>   // interrupts are disabled by interrupt/exception entry
>>>   irqentry_enter(regs)
>>>      establish_state();
>>>      // RCU is watching
>>>      arch_irqentry_enter_rcu()
>>>         mte_check_tfsr_entry();
>>>         mte_disable_tco_entry();
>>>         daif_inherit(regs);
>>>      // interrupts are still disabled
>>>           
>>> Which is equivalent versus the MTE/DAIF requirements, no?
>>
>> As above, we can't use local_daif_inherit() here because we want
>> different DAIF masking behavior for entry to interrupts and entry to
>> synchronous exceptions. While we could pass some token around to
>> determine the behaviour dynamically, that's less clear, more
>> complicated, and results in worse code being generated for something we
>> know at compile time.
> 
> I get it. Duh what a maze.
> 
>> If we can leave DAIF masked early on during irqentry_enter(), I don't
>> see why we can't leave all DAIF exceptions masked until the end of
>> irqentry_enter().
> 
> Yes. Entry is not an issue.
> 
>> I *think* what would work for us is we could split some of the exit
>> handling (including involuntary preemption) into a "prepare" step, as we
>> have for return to userspace. That way, arm64 could handle exiting
>> something like:
>>
>> 	local_irq_disable();
>> 	irqentry_exit_prepare(); // new, all generic logic
>> 	local_daif_mask();
>> 	arm64_exit_to_kernel_mode() {
>> 		...
>> 		irqentry_exit(); // ideally irqentry_exit_to_kernel_mode().
>> 		...
>> 	}
>>
>> ... and other architectures can use a combined exit_to_kernel_mode() (or
>> whatever we call that), which does both, e.g.
>>
>> 	// either noinstr, __always_inline, or a macro
>> 	void irqentry_prepare_and_exit(void)
> 
> That's a bad idea as that would require to do a full kernel rename of
> all existing irqentry_exit() users.

I see your point about the rename. However, we can avoid a tree-wide
rename by keeping the irqentry_exit() name and interface exactly as it is.
The idea is to perform an internal refactoring: split the existing logic
into two helpers (e.g., irqentry_exit_prepare() and a core helper), and
then have the original irqentry_exit() call both of them. This way,
existing users like RISC-V remain untouched, while arm64 can choose to
call the two sub-functions individually to insert the DAIF masking in
between."

> 
>> 	{
>> 		irqentry_exit_prepare();
>> 		irqentry_exit();
>> 	}
> 
> Aside of the naming that should work.
> 
> Thanks,
> 
>         tglx
> 
> 
> 
> 
>