Re: [RFC PATCH 3/4] KVM: x86: Add EOI exit bitmap inference

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/05/2015 17:23, Alex Williamson wrote:
> > Would irq_ack_notifiers be used at all with this patch set?  Resampling
> > of IOAPIC level-triggered interrupts would be implemented in userspace.
> >  For the same reason, assigned devices using legacy device assignment
> > probably would not be able to use INTX (so this feature should depend on
> > !KVM_DEVICE_ASSIGNMENT).  Add the emulated i8254 and the last user of
> > irq_ack_notifiers goes away.
> > 
> > Alex, how does VFIO do INTX resampling if you're using TCG or -machine
> > kernel_irqchip=off?  (Context: this series keeps the local APIC
> > emulation in the kernel, thus including MSI, but moves the IOAPIC
> > emualtion to userspace).
> 
> Without KVM irqchip, we use a rudimentary approach where we disable
> mmaps to the device when an interrupt occurs.  This makes us trap all
> accesses to the device.  We then handle any device access from the guest
> as a potential EOI and unmask the interrupt.  If the interrupt re-fires,
> we've at least rate-limited it.  If it doesn't, a timer eventually
> re-enables devices mmaps.  It's not very efficient, but it works (for
> both PCI and platform devices) and avoids IRQ APIs through QEMU that get
> very platform/machine specific.  Thanks,

If we move the IOAPIC back to userspace, it probably would be easier to
implement irqfd/resamplefd in userspace as well.  Let the bikeshedding
start!

Paolo