Re: [PATCH] hw/intc/ioapic: Update KVM routes before redelivering IRQ, on RTE update

[Peter Xu <peterx@redhat.com>]

On Mon, Mar 06, 2023 at 05:28:24PM +0000, David Woodhouse wrote:
> Indeed, I don't think we care about the in-kernel I/OAPIC. I don't
> think we care about the kernel knowing about e.g. "GSI #11" at all. We
> can just deliver it as MSI (for the I/OAPIC) or using KVM_INTERRUPT and
> the interrupt window as we do for the PIC. Which is why I'd happily rip
> that out and let it be delivered via the APIC intercept at 0xfeexxxxx.
> 
> The existing code which just keeps IRQ routes updated when they're
> valid is kind of OK, and well-behaved guests can function. But it isn't
> *right* in the case where they aren't valid.
> 
> What *ought* to happen is that the IOMMU should raise a fault at the
> moment the interrupt occurs, if the translation isn't valid. And we
> don't have that at all.

Right, that's definitely not ideal as an emulator.

> 
> As for why I care? I don't really *need* it, as I have everything I
> need for Xen PIRQ support already merged in 
> https://gitlab.com/qemu-project/qemu/-/commit/6096cf7877
> 
> So while the thread at
> https://lore.kernel.org/qemu-devel/aaef9961d210ac1873153bf3cf01d984708744fc.camel@infradead.org/
> was partly driven by expecting to need this for Xen PIRQ support
> (because in $DAYJOB I did those things in the other order and the PIRQ
> support ended up just being a trivial different translator like the
> IOMMU's IR)... I'd still quite like to fix it up in QEMU anyway, just
> for correctness and fidelity in the faulting cases.
> 
> We can do more efficient invalidation too, rather than blowing away the
> entire routing table every time. Just disconnect the IRQFD for the
> specific interrupts that get invalidated, and let them get fixed up
> again next time they occur.

I'm curious whether there's anything else beside the "correctness of
emulation" reason.

I would think it nice if it existed or trivial to have as what you said.  I
just don't know whether it's as easy, at least so far a new kernel
interface seems still needed, allowing a kernel irq to be paused until
being translated by QEMU from some channel we provide.

So, IMHO it's about whether the reason that "we want to have a complete
emulation of IR" can properly justify the complexity of at least the kernel
interface (I don't worry on the qemu side a lot).  After all, even if it
can completes the emulation, 99.99% of people will not use it. :(

-- 
Peter Xu