51 lines
1.9 KiB
ReStructuredText
51 lines
1.9 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=======================================
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Known limitations of CPU virtualization
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=======================================
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Whenever perfect emulation of a CPU feature is impossible or too hard, KVM
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has to choose between not implementing the feature at all or introducing
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behavioral differences between virtual machines and bare metal systems.
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This file documents some of the known limitations that KVM has in
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virtualizing CPU features.
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x86
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===
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``KVM_GET_SUPPORTED_CPUID`` issues
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----------------------------------
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x87 features
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~~~~~~~~~~~~
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Unlike most other CPUID feature bits, CPUID[EAX=7,ECX=0]:EBX[6]
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(FDP_EXCPTN_ONLY) and CPUID[EAX=7,ECX=0]:EBX]13] (ZERO_FCS_FDS) are
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clear if the features are present and set if the features are not present.
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Clearing these bits in CPUID has no effect on the operation of the guest;
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if these bits are set on hardware, the features will not be present on
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any virtual machine that runs on that hardware.
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**Workaround:** It is recommended to always set these bits in guest CPUID.
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Note however that any software (e.g ``WIN87EM.DLL``) expecting these features
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to be present likely predates these CPUID feature bits, and therefore
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doesn't know to check for them anyway.
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Nested virtualization features
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------------------------------
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TBD
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x2APIC
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------
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When KVM_X2APIC_API_USE_32BIT_IDS is enabled, KVM activates a hack/quirk that
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allows sending events to a single vCPU using its x2APIC ID even if the target
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vCPU has legacy xAPIC enabled, e.g. to bring up hotplugged vCPUs via INIT-SIPI
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on VMs with > 255 vCPUs. A side effect of the quirk is that, if multiple vCPUs
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have the same physical APIC ID, KVM will deliver events targeting that APIC ID
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only to the vCPU with the lowest vCPU ID. If KVM_X2APIC_API_USE_32BIT_IDS is
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not enabled, KVM follows x86 architecture when processing interrupts (all vCPUs
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matching the target APIC ID receive the interrupt).
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