Re: [kvm-unit-tests PATCH v1 1/4] arm64: split its-trigger test into KVM and TCG variants

Marc Zyngier
2021-04-28T16:47:01Z

On Wed, 28 Apr 2021 16:37:45 +0100,
Alex Bennée <alex.bennee@linaro.org> wrote:
>
>
> Marc Zyngier <maz@kernel.org> writes:
>
> > On Wed, 28 Apr 2021 15:00:15 +0100,
> > Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> >>
> >> I interpret that as that an INVALL guarantees that a change is
> >> visible, but it the change can become visible even without the
> >> INVALL.
> >
> > Yes. Expecting the LPI to be delivered or not in the absence of an
> > invalidate when its configuration has been altered is wrong. The
> > architecture doesn't guarantee anything of the sort.
>
> Is the underlying hypervisor allowed to invalidate and reload the
> configuration whenever it wants or should it only be driven by the
> guests requests?

The HW can do it at any time. It all depends on whether the RD has
cached this LPI configuration or not. KVM relies on the required
invalidation as a hook to reload the cached state, as it has an
infinite LPI configuration cache, while TCG doesn't have a cache at
all. Both approaches are valid implementations.

> I did consider a more nuanced variant of the test that allowed for a
> delivery pre-inval and a pass for post-inval as long as it had been
> delivered one way or another:
>
> --8<---------------cut here---------------start------------->8---
> modified arm/gic.c
> @@ -36,6 +36,7 @@ static struct gic *gic;
> static int acked[NR_CPUS], spurious[NR_CPUS];
> static int irq_sender[NR_CPUS], irq_number[NR_CPUS];
> static cpumask_t ready;
> +static bool under_tcg;
>
> static void nr_cpu_check(int nr)
> {
> @@ -687,6 +688,7 @@ static void test_its_trigger(void)
> struct its_collection *col3;
> struct its_device *dev2, *dev7;
> cpumask_t mask;
> + bool before, after;
>
> if (its_setup1())
> return;
> @@ -734,15 +736,17 @@ static void test_its_trigger(void)
> /*
> * re-enable the LPI but willingly do not call invall
> * so the change in config is not taken into account.
> - * The LPI should not hit
> + * The LPI should not hit. This does however depend on

This first point is *wrong*. From the architecture spec:

<quote>
* A change to the LPI configuration is not guaranteed to be visible
until an appropriate invalidation operation has completed:

- If one or more ITS is implemented, invalidation is performed using
the INV or INVALL command. A SYNC command completes the INV and
INVALL commands.
</quote>

*not guaranteed* means that it may fire, it may not.

> + * implementation defined behaviour - under QEMU TCG emulation
> + * it can quite correctly process the event directly.

I really don't see the point in testing IMPDEF behaviours. We should
test for architectural compliance, not for implementation choices.

M.

--
Without deviation from the norm, progress is not possible.