Re: [PATCH 5/5] arm/arm64: KVM: Initialize the vgic on-demand when injecting IRQs

Christoffer Dall
2014-12-12T11:05:05Z

On Thu, Dec 11, 2014 at 01:38:16PM +0100, Eric Auger wrote:
> On 12/11/2014 01:01 PM, Christoffer Dall wrote:
> > On Wed, Dec 10, 2014 at 01:45:50PM +0100, Eric Auger wrote:
> >> On 12/09/2014 04:44 PM, Christoffer Dall wrote:
> >>> Userspace assumes that it can wire up IRQ injections after having
> >>> created all VCPUs and after having created the VGIC, but potentially
> >>> before starting the first VCPU. This can currently lead to lost IRQs
> >>> because the state of that IRQ injection is not stored anywhere and we
> >>> don't return an error to userspace.
> >>>
> >>> We haven't seen this problem manifest itself yet,
> >> Actually we did with VFIO signaling setup before VGIC init!
> >> presumably because
> >
> > well, not with code in mainline
> >
> >>> guests reset the devices on boot, but this could cause issues with
> >>> migration and other non-standard startup configurations.
> >>>
> >>> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
> >>> ---
> >>> virt/kvm/arm/vgic.c | 9 +++++++--
> >>> 1 file changed, 7 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
> >>> index c98cc6b..feef015 100644
> >>> --- a/virt/kvm/arm/vgic.c
> >>> +++ b/virt/kvm/arm/vgic.c
> >>> @@ -1693,8 +1693,13 @@ out:
> >>> int kvm_vgic_inject_irq(struct kvm *kvm, int cpuid, unsigned int irq_num,
> >>> bool level)
> >>> {
> >>> - if (likely(vgic_ready(kvm)) &&
> >>> - vgic_update_irq_pending(kvm, cpuid, irq_num, level))
> >>> + if (unlikely(!vgic_initialized(kvm))) {
> >>> + mutex_lock(&kvm->lock);
> >>> + vgic_init(kvm);
> >>> + mutex_unlock(&kvm->lock);
> >>> + }
> >> I was previously encouraged to test the virtual interrupt controller
> >> readiness when setting irqfd up(proposal made in
> >> https://lkml.org/lkml/2014/12/3/601). I guess this becomes useless now,
> >> correct? Reviewed-by on the whole series.
> >>
> > I think we should move to your userspace explicit init for all
> > non-legacy userspace and only support gicv3 and vfio/irqfd stuff with
> > userspace explicitly initializing the vgic.
>
> Hi Christoffer,
>
> The use case I have in mind still is VFIO+irqfd:
> since we cannot preclude the user from ignoring the userspace explicit
> init and setting up VFIO signaling+irqfd before vgic init, to me there
> is a risk injection starts even before creation. Either we test
> irqchip_in_kernel in kvm_vgic_inject_irq or we must have a test when
> setting up irqfd as proposed in above patch.

yes, test if the vgic has been initialized when setting up irqfd and
return an error if not.

>
> Actually before being able to inject any virtual IRQ we weed even more:
> if virtual IRQ settings were not yet defined by the guest we do not know
> what to do with the IRQ. We must a least know whether it is level or
> edge. Current irq_cfg bitmap might be insufficient to store the info
> since it only has 2 states and by chance I use a level-sensitive IRQ and
> my QEMU pieces pay attention to that sequencing. I guess the problem is
> the same for user-space injection, isn't it?
>
that's no different from on the hardware is it? We assume the interrupt
is what it is (as per its default reset value in GICD_ICFGRn) and when
the guest boots up, it must reconfigure the interrupt.

The higher level picture here is that we don't know when the guest is
done configuring things, from some time before we run any vcpu to some
time after we've run vcpus, and there is no error return path on
injecting the IRQ to the VGIC for this sort of matter, so we just have
to cope with things in the same way that hardware does.

My guess is that no sane guests will actually depend on these interrupts
being raised/lowered before configuring the GIC will have any real
effect, as all guests should configure the gic, clear that interrupt,
unmask it, and only then care about things.

-Christoffer