Re: [PATCH 2/2] rust: add bindings for interrupt sources

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/22/24 11:30, Philippe Mathieu-Daudé wrote:
> On 22/11/24 09:32, Paolo Bonzini wrote:
>>>> +/// Interrupt sources are used by devices to pass changes to a 
>>>> boolean value to
>>>> +/// other devices (typically interrupt or GPIO controllers).  QEMU 
>>>> interrupt
>>>> +/// sources are always active-high.
>>>
>>> So 'always active-high' = true below? (Wondering about pulsation, if the
>>> true -> false transition is always correct).
>>
>> Yeah, I mean that raise uses true (or 1 :)) and lower uses false.
>> an example?
> 
> I was thinking of an active-low line where you want to pulse 1 -> 0.
> Just chiming in, not to worry about.

This is not happening at the device level, so I assume that such a line 
would not use raise/lower.  Rather, the board (which is on the interrupt 
sink side) would install a qemu_irq_invert() between the device and the 
interrupt controller or GPIO controller.

>>> Is this deliberate to restrict the Rust binding to boolean? (Maybe you
>>> envision a VectoredInterruptSource implementation for that).
>>
>> No, I simply wasn't aware of that.  I'll adjust; do you have
>> an example?
> 
> I am having hard time to find one, in particular because I
> removed one in c264c074d8 ("hw/intc: Remove TYPE_ETRAX_FS_PIC device"):

Ok, then we could put the type as a generic parameter, and use that 
parameter in InterruptSource::set().

pub struct InterruptSource<T = bool> where u32: From<T> {
     inner: BqlCell<*mut IrqState>,

     // this is only needed top ensure that T appears somehow in the
     // struct.  Random Rust type theory stuff. :)
     _marker: PhantomData<fn(&Self, T)>,
}

...

/// Send `level` to the interrupt sink.
pub fn set(&self, level: T) {
     let ptr = self.0.get();
     // SAFETY: the pointer is retrieved under the BQL and remains valid
     // until the BQL is released, which is after qemu_set_irq() is entered.
     unsafe {
         qemu_set_irq(ptr, level.into());
     }
}

and then only implement raise/lower/pulse for InterruptSource<bool>.

This is backwards compatible so we can do it either now, or later when 
needs arises.  You tell me. :)

Paolo

> See Peter's comment in https://lore.kernel.org/qemu-devel/ 
> CAFEAcA9cObnb11cSS_StbSHdP0aB6sDeqSHfjb3-qRBfy7K9Kw@mail.gmail.com/
> 
>>>> +/// Interrupt sources can only be triggered under the Big QEMU 
>>>> Lock; they are
>>>> +/// neither `Send` nor `Sync`.
>>
>> Oops, this is incorrect.  BqlCell *is* Send/Sync, but checks the
>> BQL state at run-time.
>>
>> Paolo
>>
> 
> 
>