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Diffstat (limited to 'Documentation/virt/kvm/api.rst')
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diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index cd8a58556804..2b4b64797191 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -6398,3 +6398,63 @@ 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. + +CPU topology +~~~~~~~~~~~~ + +Several CPUID values include topology information for the host CPU: +0x0b and 0x1f for Intel systems, 0x8000001e for AMD systems. Different +versions of KVM return different values for this information and userspace +should not rely on it. Currently they return all zeroes. + +If userspace wishes to set up a guest topology, it should be careful that +the values of these three leaves differ for each CPU. In particular, +the APIC ID is found in EDX for all subleaves of 0x0b and 0x1f, and in EAX +for 0x8000001e; the latter also encodes the core id and node id in bits +7:0 of EBX and ECX respectively. + +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 |