1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ======================================= 3 =======================================
4 Known limitations of CPU virtualization 4 Known limitations of CPU virtualization
5 ======================================= 5 =======================================
6 6
7 Whenever perfect emulation of a CPU feature is 7 Whenever perfect emulation of a CPU feature is impossible or too hard, KVM
8 has to choose between not implementing the fea 8 has to choose between not implementing the feature at all or introducing
9 behavioral differences between virtual machine 9 behavioral differences between virtual machines and bare metal systems.
10 10
11 This file documents some of the known limitati 11 This file documents some of the known limitations that KVM has in
12 virtualizing CPU features. 12 virtualizing CPU features.
13 13
14 x86 14 x86
15 === 15 ===
16 16
17 ``KVM_GET_SUPPORTED_CPUID`` issues 17 ``KVM_GET_SUPPORTED_CPUID`` issues
18 ---------------------------------- 18 ----------------------------------
19 19
20 x87 features 20 x87 features
21 ~~~~~~~~~~~~ 21 ~~~~~~~~~~~~
22 22
23 Unlike most other CPUID feature bits, CPUID[EA 23 Unlike most other CPUID feature bits, CPUID[EAX=7,ECX=0]:EBX[6]
24 (FDP_EXCPTN_ONLY) and CPUID[EAX=7,ECX=0]:EBX]1 24 (FDP_EXCPTN_ONLY) and CPUID[EAX=7,ECX=0]:EBX]13] (ZERO_FCS_FDS) are
25 clear if the features are present and set if t 25 clear if the features are present and set if the features are not present.
26 26
27 Clearing these bits in CPUID has no effect on 27 Clearing these bits in CPUID has no effect on the operation of the guest;
28 if these bits are set on hardware, the feature 28 if these bits are set on hardware, the features will not be present on
29 any virtual machine that runs on that hardware 29 any virtual machine that runs on that hardware.
30 30
31 **Workaround:** It is recommended to always se 31 **Workaround:** It is recommended to always set these bits in guest CPUID.
32 Note however that any software (e.g ``WIN87EM. 32 Note however that any software (e.g ``WIN87EM.DLL``) expecting these features
33 to be present likely predates these CPUID feat 33 to be present likely predates these CPUID feature bits, and therefore
34 doesn't know to check for them anyway. 34 doesn't know to check for them anyway.
35 35
36 Nested virtualization features 36 Nested virtualization features
37 ------------------------------ 37 ------------------------------
38 38
39 TBD 39 TBD
40 <<
41 x2APIC <<
42 ------ <<
43 When KVM_X2APIC_API_USE_32BIT_IDS is enabled, <<
44 allows sending events to a single vCPU using i <<
45 vCPU has legacy xAPIC enabled, e.g. to bring u <<
46 on VMs with > 255 vCPUs. A side effect of the <<
47 have the same physical APIC ID, KVM will deliv <<
48 only to the vCPU with the lowest vCPU ID. If <<
49 not enabled, KVM follows x86 architecture when <<
50 matching the target APIC ID receive the interr <<
51 <<
52 MTRRs <<
53 ----- <<
54 KVM does not virtualize guest MTRR memory type <<
55 MSRs, i.e. {RD,WR}MSR in the guest will behave <<
56 honor guest MTRRs when determining the effecti <<
57 treats all of guest memory as having Writeback <<
58 <<
59 CR0.CD <<
60 ------ <<
61 KVM does not virtualize CR0.CD on Intel CPUs. <<
62 emulates CR0.CD accesses so that loads and sto <<
63 expected, but setting CR0.CD=1 has no impact o <<
64 memory. <<
65 <<
66 Note, this erratum does not affect AMD CPUs, w <<
67 hardware, i.e. put the CPU caches into "no fil <<
68 running in the guest. <<
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