linux-kernel - [PATCH 9/9] Add cpuid.txt file

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Date:	Tue, 11 May 2010 12:17:47 -0400
From:	Glauber Costa <glommer@...hat.com>
To:	kvm@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, avi@...hat.com, zamsden@...hat.com
Subject: [PATCH 9/9] Add cpuid.txt file

This file documents cpuid bits used by KVM.

Signed-off-by: Glauber Costa <glommer@...hat.com>
---
 Documentation/kvm/cpuid.txt |   44 +++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 44 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/kvm/cpuid.txt

diff --git a/Documentation/kvm/cpuid.txt b/Documentation/kvm/cpuid.txt
new file mode 100644
index 0000000..ca1e89f
--- /dev/null
+++ b/Documentation/kvm/cpuid.txt
@@ -0,0 +1,44 @@
+KVM CPUID bits
+Glauber Costa <glommer@...hat.com>, Red Hat Inc, 2010
+=====================================================
+
+A guest running on a kvm host, can check some of its features using
+cpuid. This is not always guaranteed to work, since userspace can
+mask-out some, or even all KVM-related cpuid features before launching
+a guest.
+
+KVM cpuid functions are:
+
+function: KVM_CPUID_SIGNATURE (0x40000000)
+returns : eax = 0,
+          ebx = 0x4b4d564b,
+          ecx = 0x564b4d56,
+          edx = 0x4d.
+Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM".
+This function queries the presence of KVM cpuid leafs.
+
+
+function: define KVM_CPUID_FEATURES (0x40000001)
+returns : ebx, ecx, edx = 0
+          eax = and OR'ed group of (1 << flag), where each flags is:
+
+
+flag                               || value || meaning
+=============================================================================
+KVM_FEATURE_CLOCKSOURCE            ||     0 || kvmclock available at msrs
+                                   ||       || 0x11 and 0x12. 
+------------------------------------------------------------------------------
+KVM_FEATURE_NOP_IO_DELAY           ||     1 || not necessary to perform delays
+                                   ||       || on PIO operations.
+------------------------------------------------------------------------------
+KVM_FEATURE_MMU_OP                 ||     2 || deprecated.
+------------------------------------------------------------------------------
+KVM_FEATURE_CLOCKSOURCE2           ||     3 || kvmclock available at msrs
+                                   ||       || 0x4b564d00 and 0x4b564d01
+------------------------------------------------------------------------------
+KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will wanr if no guest-side
+                                   ||       || per-cpu warps are expected in
+                                   ||       || kvmclock.
+------------------------------------------------------------------------------
+
+
-- 
1.6.2.2

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