Documentation: KVM: add API issues section

Add a section to document all the different ways in which the KVM API sucks.

I am sure there are way more, give people a place to vent so that userspace
authors are aware.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220322110712.222449-4-pbonzini@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -7719,3 +7719,49 @@ only be invoked on a VM prior to the creation of VCPUs.
 At this time, KVM_PMU_CAP_DISABLE is the only capability.  Setting
 this capability will disable PMU virtualization for that VM.  Usermode
 should adjust CPUID leaf 0xA to reflect that the PMU is disabled.
+
+9. Known KVM API problems
+=========================
+
+In some cases, KVM's API has some inconsistencies or common pitfalls
+that userspace need to be aware of.  This section details some of
+these issues.
+
+Most of them are architecture specific, so the section is split by
+architecture.
+
+9.1. x86
+--------
+
+``KVM_GET_SUPPORTED_CPUID`` issues
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In general, ``KVM_GET_SUPPORTED_CPUID`` is designed so that it is possible
+to take its result and pass it directly to ``KVM_SET_CPUID2``.  This section
+documents some cases in which that requires some care.
+
+Local APIC features
+~~~~~~~~~~~~~~~~~~~
+
+CPU[EAX=1]:ECX[21] (X2APIC) is reported by ``KVM_GET_SUPPORTED_CPUID``,
+but it can only be enabled if ``KVM_CREATE_IRQCHIP`` or
+``KVM_ENABLE_CAP(KVM_CAP_IRQCHIP_SPLIT)`` are used to enable in-kernel emulation of
+the local APIC.
+
+The same is true for the ``KVM_FEATURE_PV_UNHALT`` paravirtualized feature.
+
+CPU[EAX=1]:ECX[24] (TSC_DEADLINE) is not reported by ``KVM_GET_SUPPORTED_CPUID``.
+It can be enabled if ``KVM_CAP_TSC_DEADLINE_TIMER`` is present and the kernel
+has enabled in-kernel emulation of the local APIC.
+
+Obsolete ioctls and capabilities
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+KVM_CAP_DISABLE_QUIRKS does not let userspace know which quirks are actually
+available.  Use ``KVM_CHECK_EXTENSION(KVM_CAP_DISABLE_QUIRKS2)`` instead if
+available.
+
+Ordering of KVM_GET_*/KVM_SET_* ioctls
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+TBD