From: Paolo Bonzini <pbonzini@xxxxxxxxxx> [ 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@xxxxxxxxxx> Message-Id: <20220322110712.222449-4-pbonzini@xxxxxxxxxx> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- Documentation/virt/kvm/api.rst | 46 ++++++++++++++++++++++++++++++++++ 1 file changed, 46 insertions(+) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index cd8a58556804..d807994360d4 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -6398,3 +6398,49 @@ When enabled, KVM will disable paravirtual features provided to the guest according to the bits in the KVM_CPUID_FEATURES CPUID leaf (0x40000001). Otherwise, a guest may use the paravirtual features regardless of what has actually been exposed through the CPUID leaf. + +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 -- 2.35.1
