1 .. SPDX-License-Identifier: GPL-2.0
2
3 =======================================
4 Known limitations of CPU virtualization
5 =======================================
6
7 Whenever perfect emulation of a CPU feature is
8 has to choose between not implementing the fea
9 behavioral differences between virtual machine
10
11 This file documents some of the known limitati
12 virtualizing CPU features.
13
14 x86
15 ===
16
17 ``KVM_GET_SUPPORTED_CPUID`` issues
18 ----------------------------------
19
20 x87 features
21 ~~~~~~~~~~~~
22
23 Unlike most other CPUID feature bits, CPUID[EA
24 (FDP_EXCPTN_ONLY) and CPUID[EAX=7,ECX=0]:EBX]1
25 clear if the features are present and set if t
26
27 Clearing these bits in CPUID has no effect on
28 if these bits are set on hardware, the feature
29 any virtual machine that runs on that hardware
30
31 **Workaround:** It is recommended to always se
32 Note however that any software (e.g ``WIN87EM.
33 to be present likely predates these CPUID feat
34 doesn't know to check for them anyway.
35
36 Nested virtualization features
37 ------------------------------
38
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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