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

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2015-05-14 at 17:46 +0200, Paolo Bonzini wrote:
> 
> 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!

The current solution is merely functional with the benefit of being
architecture and machine-type independent.  If someone actually cared
about vfio assigned device performance without KVM irqchip, we'd
certainly need a more deterministic and efficient EOI path.  Thanks,

Alex