Re: [PATCH v13 06/13] irqchip: Add RISC-V incoming MSI controller early driver

[Björn Töpel]

Anup Patel <apatel@xxxxxxxxxxxxxxxx> writes:

> On Wed, Feb 21, 2024 at 5:29 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
>>
>> Anup Patel <apatel@xxxxxxxxxxxxxxxx> writes:
>>
>> >> > +void imsic_vector_mask(struct imsic_vector *vec)
>> >> > +{
>> >> > +     struct imsic_local_priv *lpriv;
>> >> > +
>> >> > +     lpriv = per_cpu_ptr(imsic->lpriv, vec->cpu);
>> >> > +     if (WARN_ON(&lpriv->vectors[vec->local_id] != vec))
>> >> > +             return;
>> >> > +
>> >> > +     /*
>> >> > +      * This function is called through Linux irq subsystem with
>> >> > +      * irqs disabled so no need to save/restore irq flags.
>> >> > +      */
>> >> > +
>> >> > +     raw_spin_lock(&lpriv->lock);
>> >> > +
>> >> > +     vec->enable = false;
>> >> > +     bitmap_set(lpriv->dirty_bitmap, vec->local_id, 1);
>> >> > +     __imsic_remote_sync(lpriv, vec->cpu);
>> >> > +
>> >> > +     raw_spin_unlock(&lpriv->lock);
>> >> > +}
>> >>
>> >> Really nice that you're using a timer for the vector affinity change,
>> >> and got rid of the special/weird IMSIC/sync IPI. Can you really use a
>> >> timer for mask/unmask? That makes the mask/unmask operation
>> >> asynchronous!
>> >>
>> >> That was what I was trying to get though with this comment:
>> >> https://lore.kernel.org/linux-riscv/87sf24mo1g.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
>> >>
>> >> Also, using the smp_* IPI functions, you can pass arguments, so you
>> >> don't need the dirty_bitmap tracking the changes.
>> >
>> > The mask/unmask operations are called with irqs disabled so if
>> > CPU X does synchronous IPI to another CPU Y from mask/unmask
>> > operation then while CPU X is waiting for IPI to complete it cannot
>> > receive IPI from other CPUs which can lead to crashes and stalls.
>> >
>> > In general, we should not do any block/busy-wait work in
>> > mask/unmask operation of an irqchip driver.
>>
>> Hmm, OK. Still, a bit odd that when the .irq_mask callback return, the
>> masking is not actually completed.
>>
>> 1. CPU 0 tries to mask an interrupt tried to CPU 1.
>> 2. The timer is queued on CPU 1.
>> 3. The call irq_mask returns on CPU 0
>> 4. ...the irq is masked at some future point, determined by the callback
>>    at CPU 1
>>
>> Is that the expected outcome?
>
> Yes, that's right.
>
>>
>> There are .irq_mask implementation that does seem to go at length
>> (blocking) to perform the mask, e.g.: gic_mask_irq() which calls
>> gic_{re,}dist_wait_for_rwp that have sleep/retry loops. The GIC3 ITS
>> code has similar things going on.
>
> The gic_{re,}dist_wait_for_rwp() polls on a HW register for completion
> which will certainly complete in a predictable time whereas waiting
> for IPI to be executed by another CPU is not predictable and fragile.
>
>>
>> I'm not saying you're wrong, I'm just trying to wrap my head around the
>> masking semantics.
>>
>> > The AIA IMSIC spec allows setting ID pending bit using MSI write
>> > irrespective whether ID is enabled or not but the interrupt will be
>> > taken only after ID is enabled. In other words, there will be no
>> > loss of interrupt with delayed mask/unmask using async IPI or
>> > lazy timer.
>>
>> No loss, but we might *get* an interrupt when we explicitly asked not to
>> get any. Maybe that's ok?
>>
>
> The delayed spurious interrupt after masking is avoided by additional
> masking at the source of interrupt. For wired-to-MSI interrupts, we have
> additional masking on the APLIC MSI-mode. For PCI MSI interrupts, we
> have additional masking at PCI device level using pci_msi_mask_irq().

Thanks for the clarifications, Anup! Much appreciated!