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Documentation: KVM: add API issues section

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[ Upstream commit cde363ab7ca7aea7a853851cd6a6745a9e1aaf5e ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Paolo Bonzini
2022-03-22 12:07:12 +01:00
committed by Greg Kroah-Hartman
parent b8f3b3cffb
commit ee16841134
1 changed files with 46 additions and 0 deletions
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46
Documentation/virt/kvm/api.rst
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@@ -7265,3 +7265,49 @@ The argument to KVM_ENABLE_CAP is also a bitmask, and must be a subset
of the result of KVM_CHECK_EXTENSION. KVM will forward to userspace
the hypercalls whose corresponding bit is in the argument, and return
ENOSYS for the others.
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