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Documentation / virtual / kvm / cpuid.txt




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Based on kernel version 3.9. Page generated on 2013-05-02 23:16 EST.

1	KVM CPUID bits
2	Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010
3	=====================================================
4	
5	A guest running on a kvm host, can check some of its features using
6	cpuid. This is not always guaranteed to work, since userspace can
7	mask-out some, or even all KVM-related cpuid features before launching
8	a guest.
9	
10	KVM cpuid functions are:
11	
12	function: KVM_CPUID_SIGNATURE (0x40000000)
13	returns : eax = 0x40000001,
14	          ebx = 0x4b4d564b,
15	          ecx = 0x564b4d56,
16	          edx = 0x4d.
17	Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM".
18	The value in eax corresponds to the maximum cpuid function present in this leaf,
19	and will be updated if more functions are added in the future.
20	Note also that old hosts set eax value to 0x0. This should
21	be interpreted as if the value was 0x40000001.
22	This function queries the presence of KVM cpuid leafs.
23	
24	
25	function: define KVM_CPUID_FEATURES (0x40000001)
26	returns : ebx, ecx, edx = 0
27	          eax = and OR'ed group of (1 << flag), where each flags is:
28	
29	
30	flag                               || value || meaning
31	=============================================================================
32	KVM_FEATURE_CLOCKSOURCE            ||     0 || kvmclock available at msrs
33	                                   ||       || 0x11 and 0x12.
34	------------------------------------------------------------------------------
35	KVM_FEATURE_NOP_IO_DELAY           ||     1 || not necessary to perform delays
36	                                   ||       || on PIO operations.
37	------------------------------------------------------------------------------
38	KVM_FEATURE_MMU_OP                 ||     2 || deprecated.
39	------------------------------------------------------------------------------
40	KVM_FEATURE_CLOCKSOURCE2           ||     3 || kvmclock available at msrs
41	                                   ||       || 0x4b564d00 and 0x4b564d01
42	------------------------------------------------------------------------------
43	KVM_FEATURE_ASYNC_PF               ||     4 || async pf can be enabled by
44	                                   ||       || writing to msr 0x4b564d02
45	------------------------------------------------------------------------------
46	KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
47	                                   ||       || per-cpu warps are expected in
48	                                   ||       || kvmclock.
49	------------------------------------------------------------------------------
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