Hi Alex,
On Fri, 21 May 2021 18:01:05 +0100,
Alexandru Elisei <alexandru.elisei@xxxxxxx> wrote:
>
> Hi Marc,
>
> On 5/10/21 2:48 PM, Marc Zyngier wrote:
> > The vGIC, as architected by ARM, allows a virtual interrupt to
> > trigger the deactivation of a physical interrupt. This allows
> > the following interrupt to be delivered without requiring an exit.
>
> If I got this right, the AIC doesn't implement/ignores the
s/AIC/M1 CPU/
> ICH_LR_EL2.HW bit. Does it mean that the CPU IF behaves as if HW =
> 0b0, meaning it asserts a maintenance interrupt on virtual interrupt
> deactivation when ICH_LR_EL2.EOI = 0b1? I assume that's the case,
> just double checking.
Yes, that's what it looks like.
> I am wondering what would happen if we come across an interrupt
> controller where the CPU IF cannot assert a maintenance interrupt at
> all and we rely on the EOI bit to take us out of the guest to
> deactivate the HW interrupt.
That'd be broken, and we wouldn't be able to support such an
implementation, at least not in configuration such as CPU isolation.
> I have to say that it looks a bit strange to start relying on the
> maintenance interrupt to emulate interrupt deactivate for hardware
> interrupts, but at the same timer allowing an interrupt controller
> without a maintenance interrupt.
We are not allowing a vGIC without a maintenance interrupt. We are
actively mandating it. The M1 does have a working per-CPU maintenance
interrupt. It just isn't wired into an interrupt controller, which
means we can't mask it. But it is otherwise perfectly functional.
> Other than that, this idea sounds like the best thing to do
> considering the circumstances, I certainly can't think of anything
> better.
>
> >
> > However, some implementations have choosen not to implement this,
> > meaning that we will need some unsavoury workarounds to deal with this.
> >
> > On detecting such a case, taint the kernel and spit a nastygram.
> > We'll deal with this in later patches.
> >
> > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > ---
> > arch/arm64/kvm/vgic/vgic-init.c | 10 ++++++++++
> > include/kvm/arm_vgic.h | 3 +++
> > include/linux/irqchip/arm-vgic-info.h | 2 ++
> > 3 files changed, 15 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
> > index 9fd23f32aa54..5b06a9970a57 100644
> > --- a/arch/arm64/kvm/vgic/vgic-init.c
> > +++ b/arch/arm64/kvm/vgic/vgic-init.c
> > @@ -524,6 +524,16 @@ int kvm_vgic_hyp_init(void)
> > if (!gic_kvm_info)
> > return -ENODEV;
> >
> > + /*
> > + * If we get one of these oddball non-GICs, taint the kernel,
> > + * as we have no idea of how they *really* behave.
> > + */
> > + if (gic_kvm_info->no_hw_deactivation) {
> > + kvm_info("Non-architectural vgic, tainting kernel\n");
> > + add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
> > + kvm_vgic_global_state.no_hw_deactivation = true;
> > + }
>
> IMO, since this means we're going to rely even more on the
> maintenance interrupt (not just for software emulation of level
> sensitive interrupts), I think there should be some sort of
> dependency on having something that resembles a working maintenance
> interrupt.
But the timer interrupt is exactly a SW emulation of a level sensitive
interrupt in this context. And the maintenance interrupt is still
required to use the vGIC.
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
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