Hi Christoffer,
On 07/12/17 11:54, Christoffer Dall wrote:
> The timer was modeled after a strict idea of modelling an interrupt line
> level in software, meaning that only transitions in the level needed to
> be reported to the VGIC. This works well for the timer, because the
> arch timer code is in complete control of the device and can track the
> transitions of the line.
>
> However, as we are about to support using the HW bit in the VGIC not
> just for the timer, but also for VFIO which cannot track transitions of
> the interrupt line, we have to decide on an interface for level
> triggered mapped interrupts to the GIC, which both the timer and VFIO
> can use.
>
> VFIO only sees an asserting transition of the physical interrupt line,
> and tells the VGIC when that happens. That means that part of the
> interrupt flow is offloaded to the hardware.
>
> To use the same interface for VFIO devices and the timer, we therefore
> have to change the timer (we cannot change VFIO because it doesn't know
> the details of the device it is assigning to a VM).
>
> Luckily, changing the timer is simple, we just need to stop 'caching'
> the line level, but instead let the VGIC know the state of the timer
> every time there is a potential change in the line level, and when the
> line level should be asserted from the timer ISR. The VGIC can ignore
> extra notifications using its validate mechanism.
I was confused by the fact we say we stop caching the line level but
vtimer->irq.level still exists, is updated in the vtimer host ISR and
kvm_timer_update_state() and read in many places.
I feel difficult to figure out if each time we use the vtimer->irq.level
value it is safe to use it.
Also for the validate() to succeed we need the vgic irq->line_level to
to be 0. I understand this is properly handled for mapped level irqs in
next patch which does that on the populate_lr. However I currently fail
to understand why the timer level sensitive mapped IRQ does not require
the next patch to work.
Thanks
Eric
>
> Reviewed-by: Andre Przywara <andre.przywara@xxxxxxx>
> Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
> ---
> virt/kvm/arm/arch_timer.c | 20 +++++++++++++-------
> 1 file changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index 4151250ce8da..dd5aca05c500 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -99,11 +99,9 @@ static irqreturn_t kvm_arch_timer_handler(int irq, void *dev_id)
> }
> vtimer = vcpu_vtimer(vcpu);
>
> - if (!vtimer->irq.level) {
> - vtimer->cnt_ctl = read_sysreg_el0(cntv_ctl);
> - if (kvm_timer_irq_can_fire(vtimer))
> - kvm_timer_update_irq(vcpu, true, vtimer);
> - }
> + vtimer->cnt_ctl = read_sysreg_el0(cntv_ctl);
> + if (kvm_timer_irq_can_fire(vtimer))
> + kvm_timer_update_irq(vcpu, true, vtimer);
>
> if (unlikely(!irqchip_in_kernel(vcpu->kvm)))
> kvm_vtimer_update_mask_user(vcpu);
> @@ -324,12 +322,20 @@ static void kvm_timer_update_state(struct kvm_vcpu *vcpu)
> struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
> struct arch_timer_context *ptimer = vcpu_ptimer(vcpu);
> + bool level;
>
> if (unlikely(!timer->enabled))
> return;
>
> - if (kvm_timer_should_fire(vtimer) != vtimer->irq.level)
> - kvm_timer_update_irq(vcpu, !vtimer->irq.level, vtimer);
> + /*
> + * The vtimer virtual interrupt is a 'mapped' interrupt, meaning part
> + * of its lifecycle is offloaded to the hardware, and we therefore may
> + * not have lowered the irq.level value before having to signal a new
> + * interrupt, but have to signal an interrupt every time the level is
> + * asserted.
> + */
> + level = kvm_timer_should_fire(vtimer);
> + kvm_timer_update_irq(vcpu, level, vtimer);
>
> if (kvm_timer_should_fire(ptimer) != ptimer->irq.level)
> kvm_timer_update_irq(vcpu, !ptimer->irq.level, ptimer);
>
