Hi Eric, > -----Original Message----- > From: Dmytro Maluka <dmy@xxxxxxxxxxxx> > Sent: Thursday, July 7, 2022 2:16 AM > To: eric.auger@xxxxxxxxxx; Micah Morton <mortonm@xxxxxxxxxxxx> > Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>; > kvm@xxxxxxxxxxxxxxx; Christopherson,, Sean <seanjc@xxxxxxxxxx>; > Paolo Bonzini <pbonzini@xxxxxxxxxx>; Liu, Rong L > <rong.l.liu@xxxxxxxxx>; Tomasz Nowicki <tn@xxxxxxxxxxxx>; Grzegorz > Jaszczyk <jaz@xxxxxxxxxxxx>; Dmitry Torokhov <dtor@xxxxxxxxxx> > Subject: Re: Add vfio-platform support for ONESHOT irq forwarding? > > Hi Eric, > > On 7/7/22 10:25 AM, Eric Auger wrote: > >> Again, this doesn't seem to be true. Just as explained in my above > >> reply to Alex, the guest deactivates (EOI) the vIRQ already after the > >> completion of the vIRQ hardirq handler, not the vIRQ thread. > >> > >> So VFIO unmask handler gets called too early, before the interrupt > >> gets serviced and acked in the vIRQ thread. > > Fair enough, on vIRQ hardirq handler the physical IRQ gets unmasked. > > This event occurs on guest EOI, which triggers the resamplefd. But what > > is the state of the vIRQ? Isn't it stil masked until the vIRQ thread > > completes, preventing the physical IRQ from being propagated to the > guest? > > Even if vIRQ is still masked by the time when > vfio_automasked_irq_handler() signals the eventfd (which in itself is > not guaranteed, I guess), I believe KVM is buffering this event, so > after the vIRQ is unmasked, this new IRQ will be injected to the guest > anyway. > > >> It seems the obvious fix is to postpone sending irq ack notifications > >> in KVM from EOI to unmask (for oneshot interrupts only). Luckily, we > >> don't need to provide KVM with the info that the given interrupt is > >> oneshot. KVM can just find it out from the fact that the interrupt is > >> masked at the time of EOI. > > you mean the vIRQ right? > > Right. > > > Before going further and we invest more time in that thread, please > > could you give us additional context info and confidence > > in/understanding of the stakes. This thread is from Jan 2021 and was > > discontinued for a while. vfio-platform currently only is enabled on > ARM > > and maintained for very few devices which properly implement reset > > callbacks and duly use an underlying IOMMU. Do you have more questions about this issue after following info and POC from Dmytro? I agree that we tried to extend the vfio infrastructure to x86 and a few more devices which may not "traditionally" supported by current vfio implementation. However if we view vfio as a general infrastructure to be used for pass-thru devices (this is what we intend to do, implementation may vary), Oneshot interrupt is not properly handled. >From this discussion when oneshot interrupt is first upstreamed: https://lkml.iu.edu/hypermail/linux/kernel/0908.1/02114.html it says: "... we need to keep the interrupt line masked until the threaded handler has executed. ... The interrupt line is unmasked after the thread handler function has been executed." using today's vfio architecture, (physical) interrupt line is unmasked by vfio after EOI introduced vmexit, instead after the threaded function has been executed (or in x86 world, when virtual interrupt is unmasked): this totally violates how oneshot irq should be used. We have a few internal discussions and we couldn't find a solution which are both correct and efficient. But at least we can target a "correct" solution first and that will help us resolve bugs we have on our products now. > Sure. We are not really using vfio-platform for the devices we are > concerned with, since those are not DMA capable devices, and some of > them are not really platform devices but I2C or SPI devices. Instead we > are using (hopefully temporarily) Micah's module for forwarding > arbitrary IRQs [1][2] which mostly reimplements the VFIO irq forwarding > mechanism. > > Also with a few simple hacks I managed to use vfio-platform for the same > thing (just as a PoC) and confirmed, unsurprisingly, that the problems > with oneshot interrupts are observed with vfio-platform as well. > > [1] > https://chromium.googlesource.com/chromiumos/third_party/kernel/+/ > refs/heads/chromeos-5.10-manatee/virt/lib/platirqforward.c > > [2] > https://lkml.kernel.org/kvm/CAJ- > EccPU8KpU96PM2PtroLjdNVDbvnxwKwWJr2B+RBKuXEr7Vw@mail.gmail > .com/T/ > > Thanks, > Dmytro
