On Thu, Jul 07, 2022, Dmytro Maluka wrote: > Hi Sean, > > On 7/6/22 10:39 PM, Sean Christopherson wrote: > > On Wed, Jul 06, 2022, Dmytro Maluka wrote: > > > This is not a problem on native, since for oneshot irq we keep the interrupt > > > masked until the thread exits, so that the EOI at the end of hardirq doesn't > > > result in immediate re-assert. In vfio + KVM case, however, the host doesn't > > > check that the interrupt is still masked in the guest, so > > > vfio_platform_unmask() is called regardless. > > > > Isn't not checking that an interrupt is unmasked the real bug? Fudging around vfio > > (or whatever is doing the premature unmasking) bugs by delaying an ack notification > > in KVM is a hack, no? > > Yes, not checking that an interrupt is unmasked is IMO a bug, and my patch > actually adds this missing checking, only that it adds it in KVM, not in > VFIO. :) > > Arguably it's not a bug that VFIO is not checking the guest interrupt state > on its own, provided that the resample notification it receives is always a > notification that the interrupt has been actually acked. That is the > motivation behind postponing ack notification in KVM in my patch: it is to > ensure that KVM "ack notifications" are always actual ack notifications (as > the name suggests), not just "eoi notifications". But EOI is an ACK. It's software saying "this interrupt has been consumed". > That said, your idea of checking the guest interrupt status in VFIO (or > whatever is listening on the resample eventfd) makes sense to me too. The > problem, though, is that it's KVM that knows the guest interrupt status, so > KVM would need to let VFIO/whatever know it somehow. (I'm assuming we are > focusing on the case of KVM kernel irqchip, not userspace or split irqchip.) > So do you have in mind adding something like "maskfd" and "unmaskfd" to KVM > IRQFD interface, in addition to resamplefd? If so, I'm actually in favor of > such an idea, as I think it would be also useful for other purposes, > regardless of oneshot interrupts. Unless I'm misreading things, KVM already provides a mask notifier, irqfd just needs to be wired up to use kvm_(un)register_irq_mask_notifier().
