On 31/01/18 12:14, Christoffer Dall wrote:
> When introducing support for irqchip in userspace we needed a way to
> mask the timer signal to prevent the guest continuously exiting due to a
> screaming timer.
>
> We did this by disabling the corresponding percpu interrupt on the
> host interrupt controller, because we cannot rely on the host system
> having a GIC, and therefore cannot make any assumptions about having an
> active state to hide the timer signal.
>
> Unfortunately, when introducing this feature, it became entirely
> possible that a VCPU which belongs to a VM that has a userspace irqchip
> can disable the vtimer irq on the host on some physical CPU, and then go
> away without ever enabling the vtimer irq on that physical CPU again.
>
> This means that using irqchips in userspace on a system that also
> supports running VMs with an in-kernel GIC can prevent forward progress
> from in-kernel GIC VMs.
>
> Later on, when we started taking virtual timer interrupts in the arch
> timer code, we would also leave this timer state active for userspace
> irqchip VMs, because we leave it up to a VGIC-enabled guest to
> deactivate the hardware IRQ using the HW bit in the LR.
>
> Both issues are solved by only using the enable/disable trick on systems
> that do not have a host GIC which supports the active state, because all
> VMs on such systems must use irqchips in userspace. Systems that have a
> working GIC with support for an active state use the active state to
> mask the timer signal for both userspace and in-kernel irqchips.
>
> Cc: Alexander Graf <agraf@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.12+
> Fixes: d9e139778376 ("KVM: arm/arm64: Support arch timers with a userspace gic")
> Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
Reviewed-by: Marc Zyngier <marc.zyngier@xxxxxxx>
M.
--
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