Re: [RFC PATCH-for-9.1? 1/2] target/arm: Move qmp_query_gic_capabilities() to hw/intc/

[Markus Armbruster]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Thu, 8 Aug 2024 at 10:02, Markus Armbruster <armbru@redhat.com> wrote:
>>
>> Peter Maydell <peter.maydell@linaro.org> writes:
>>
>> > On Thu, 8 Aug 2024 at 05:32, Markus Armbruster <armbru@redhat.com> wrote:
>> >>
>> >> Peter Maydell <peter.maydell@linaro.org> writes:
>> >>
>> >> [...]
>> >>
>> >> > The QAPI command which this code is implementing is
>> >> > also (a) target-specific and (b) unfortunately
>> >> > designed so that it doesn't get passed a particular
>> >> > CPU or particular device to query, it's just assumed
>> >> > to be a part of the whole simulation.
>> >>
>> >> We can fix (b) if we care: add a suitable optional argument, default to
>> >> the sole GIC in the system, fail if there's more than one.  I assume we
>> >> have no machines with more than one now.
>> >
>> > The exynos4210 SoC (board types 'nuri', 'smdkc210') has
>> > two GICs. (It's a rather odd design -- there's the
>> > interrupt controller that's part of the main CPU
>> > cluster, and then they used a second "external" GIC
>> > that feeds into that one.)
>>
>> Then "fail if there's more than one" would be an incompatible change for
>> this machine.
>>
>> If the two GICs have identical capabilities, it doesn't matter to which
>> of the two query-gic-capabilities technically applies.
>>
>> Else, it matters, and we have an interface issue.  Do we?
>
> It's not possible to use KVM with that machine type, so the
> question is a bit moot. (This also indicates that the
> interface is not very helpful -- it purports to tell the
> management layer whether it can use an accelerated in-kernel
> GIC, but because it doesn't specifiy the board type there's
> no way to provide an accurate answer. It would be useful
> to know exactly what libvirt/etc actually use this for...)

I had a look at libvirt.

It executes query-gic-capabilities only for certain ARM architectures,
namely

    qemuCaps->arch == VIR_ARCH_AARCH64 ||
    qemuCaps->arch == VIR_ARCH_ARMV6L ||
    qemuCaps->arch == VIR_ARCH_ARMV7L

This is virQEMUCapsProbeQMPGICCapabilities().  It stores the result of
query-gic-capabilities in a virGICCapability array.

The only use of the array I can see is in
virQEMUCapsSupportsGICVersion().  It returns true when one of the array
entries matches its version argument and supports its virtType argument.
Two users:

* virQEMUCapsFillDomainFeatureGICCaps() uses it to find a supported GIC
  version.  I believe this is then stored in domain XML.

* qemuDomainDefEnableDefaultFeatures() uses it to pick a default GIC
  version.  I believe this is then used to set machine property
  "gic-version".