Re: [Qemu-devel] Question about the vfio device interrupt

[Li Qiang]

Alex Williamson <address@hidden> 于2019年5月15日周三 下午10:54写道：

> On Wed, 15 May 2019 17:27:52 +0300
> Maxim Levitsky <address@hidden> wrote:
>
> > Hi!
> >
> > Let me add my 0.2 cents to this discussion by sharing my summary of INTx
> emulation in VFIO:
> >
> > COMMON:
> > * real interrupt is signaled in the kernel
> > * vfio-pci kernel interrupt hanlder (vfio_intx_handler), masks the INTx
> flag
> > * vfio-pci kernel interrupt handler triggers an eventfd, if the
> interrupt is not virtually intx masked
> >   (an eventfd which was registred for VFIO_IRQ_SET_DATA_EVENTFD |
> VFIO_IRQ_SET_ACTION_TRIGGER)
> >
> >
> > KERNEL INTERRUPT CONTROLLER CASE:
> >
> > * the vfio-pci eventfd is listened by the kvm (set by KVM_IRQFD ioctl),
> and delivered to guest ioapic
> >   (the setup is done by qemu in vfio_intx_enable_kvm, the same eventfd
> is given to vfio-pci and to kvm)
> > * when guest writes eoi, an 'resamplefd' is (which is registered by
> vfio_intx_enable_kvm too)
> >   is triggered.
> >   the vfio_intx_enable_kvm tells the vfio pci module
> (VFIO_IRQ_SET_DATA_EVENTFD | VFIO_IRQ_SET_ACTION_UNMASK)
> >   to listen to that 'resamplefd' and unmask the interrupt
> >   (here too, a same eventfd is given to kvm and to vfio-pci, but here
> the kvm signals it, and vfio-pci listens to it).
> >
> >
> > USERSPACE QEMU CASE:
> >
> >   (only happens in short period while vfio_intx_enable is running and
> before it calles the vfio_intx_enable_kvm)
> >   (or when vfio_intx_enable_kvm fails)
> >
> > * the vfio-pci eventfd is listened by the qemu (vfio_intx_interrupt)
> > * the vfio_intx_interrupt (in qemu)
> >       * masks the interrupt using kernel vfio interface
> >       * unmaps all the device bars and sets a timer to map them again
> >       * sends the interrupt down the qemu stack till it is reinjected to
> the guest
> > * any bar access now goes to qemu via EPT fault
> (vfio_region_read/vfio_region_write)
> > * the bar access
> >       * triggers (vfio_eoi->vfio_intx_eoi) which unmasks (using
> vfio-pci) the interrupt
> >         the idea is that first few bar accesses should ack the interrupt
> and thus avoid flood
> >         but some interrupts could still happen (one per bar access)
>
> Full points :)  Thanks,
>


Thanks very much Alex and Maxim!
I appreciate it.

Thanks,
Li Qiang




>
> Alex
>