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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: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220322110712.222449-4-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2022-03-22 12:07:12 +01:00
parent 45016721de
commit cde363ab7c
1 changed files with 46 additions and 0 deletions
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Documentation/virt/kvm/api.rst
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@ -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 At this time, KVM_PMU_CAP_DISABLE is the only capability. Setting
this capability will disable PMU virtualization for that VM. Usermode this capability will disable PMU virtualization for that VM. Usermode
should adjust CPUID leaf 0xA to reflect that the PMU is disabled. 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