On Mon, May 10, 2021 at 02:48:18PM +0100, Marc Zyngier wrote:
> As it turns out, not all the interrupt controllers are able to
> expose a vGIC maintenance interrupt as a distrete signal.
> And to be fair, it doesn't really matter as all we require is
> for *something* to kick us out of guest mode out way or another.
>
> On systems that do not expose a maintenance interrupt as such,
> there are two outcomes:
>
> - either the virtual CPUIF does generate an interrupt, and
> by the time we are back to the host the interrupt will have long
> been disabled (as we set ICH_HCR_EL2.EN to 0 on exit). In this case,
> interrupt latency is as good as it gets.
>
> - or some other event (physical timer) will take us out of the guest
> anyway, and the only drawback is a bad interrupt latency.
IIRC we won't have a a guaranteed schedular tick for NO_HZ_FULL, so in
that case we'll either need to set a period software maintenance
interrupt, or reject this combination at runtime (either when trying to
isolate the dynticks CPUs, or when trying to create a VM).
Otherwise, it's very likely that something will take us out of the guest
from time to time, but we won't have a strict guarantee (e.g. if all
guest memory is pinned).
Thanks,
Mark.
>
> So let's be tolerant to the lack of maintenance interrupt, and just let
> the user know that their mileage may vary...
>
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> ---
> arch/arm64/kvm/vgic/vgic-init.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
> index 2fdb65529594..9fd23f32aa54 100644
> --- a/arch/arm64/kvm/vgic/vgic-init.c
> +++ b/arch/arm64/kvm/vgic/vgic-init.c
> @@ -524,11 +524,6 @@ int kvm_vgic_hyp_init(void)
> if (!gic_kvm_info)
> return -ENODEV;
>
> - if (!gic_kvm_info->maint_irq) {
> - kvm_err("No vgic maintenance irq\n");
> - return -ENXIO;
> - }
> -
> switch (gic_kvm_info->type) {
> case GIC_V2:
> ret = vgic_v2_probe(gic_kvm_info);
> @@ -552,6 +547,11 @@ int kvm_vgic_hyp_init(void)
> if (ret)
> return ret;
>
> + if (!kvm_vgic_global_state.maint_irq) {
> + kvm_err("No maintenance interrupt available, fingers crossed...\n");
> + return 0;
> + }
> +
> ret = request_percpu_irq(kvm_vgic_global_state.maint_irq,
> vgic_maintenance_handler,
> "vgic", kvm_get_running_vcpus());
> --
> 2.29.2
>
