| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ======================================= |
| Known limitations of CPU virtualization |
| ======================================= |
| |
| Whenever perfect emulation of a CPU feature is impossible or too hard, KVM |
| has to choose between not implementing the feature at all or introducing |
| behavioral differences between virtual machines and bare metal systems. |
| |
| This file documents some of the known limitations that KVM has in |
| virtualizing CPU features. |
| |
| x86 |
| === |
| |
| ``KVM_GET_SUPPORTED_CPUID`` issues |
| ---------------------------------- |
| |
| x87 features |
| ~~~~~~~~~~~~ |
| |
| Unlike most other CPUID feature bits, CPUID[EAX=7,ECX=0]:EBX[6] |
| (FDP_EXCPTN_ONLY) and CPUID[EAX=7,ECX=0]:EBX]13] (ZERO_FCS_FDS) are |
| clear if the features are present and set if the features are not present. |
| |
| Clearing these bits in CPUID has no effect on the operation of the guest; |
| if these bits are set on hardware, the features will not be present on |
| any virtual machine that runs on that hardware. |
| |
| **Workaround:** It is recommended to always set these bits in guest CPUID. |
| Note however that any software (e.g ``WIN87EM.DLL``) expecting these features |
| to be present likely predates these CPUID feature bits, and therefore |
| doesn't know to check for them anyway. |
| |
| Nested virtualization features |
| ------------------------------ |
| |
| TBD |
