[PATCH 3/3] Documentation: KVM: Clarify that KVM_CAP_MAX_VCPU_ID is exclusive

[Dmytro Maluka <dmaluka@chromium.org>]

The KVM_CAP_MAX_VCPU_ID capability value is exclusive, i.e. valid APIC
ID values are only values below this value, not including it. Its
documentation doesn't make that clear, and even suggests otherwise.
Fix it and explicitly state that it is exclusive.

Signed-off-by: Dmytro Maluka <dmaluka@chromium.org>
---
 Documentation/virt/kvm/api.rst | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index e3003a241d5b..dab9a28c0fb5 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8593,21 +8593,24 @@ KVM_X86_QUIRK_NESTED_SVM_SHARED_PAT        By default, KVM for nested SVM guests
 
 :Architectures: x86
 :Target: VM
-:Parameters: args[0] - maximum APIC ID value set for current VM
+:Parameters: args[0] - maximum APIC ID value (plus one) set for current VM
 :Returns: 0 on success, -EINVAL if args[0] is beyond KVM_MAX_VCPU_IDS
           supported in KVM or if it has been set.
 
-This capability allows userspace to specify maximum possible APIC ID
+This capability allows userspace to specify a limit on APIC ID values
 assigned for current VM session prior to the creation of vCPUs, saving
 memory for data structures indexed by the APIC ID.  Userspace is able
 to calculate the limit to APIC ID values from designated
 CPU topology.
 
-The value can be changed only until KVM_ENABLE_CAP is set to a nonzero
+The limit is not inclusive, i.e. the maximum possible APIC ID value is
+args[0] - 1.
+
+The capability value can only be changed until it is set to a nonzero
 value or until a vCPU is created.  Upon creation of the first vCPU,
 if the value was set to zero or KVM_ENABLE_CAP was not invoked, KVM
-uses the return value of KVM_CHECK_EXTENSION(KVM_CAP_MAX_VCPU_ID) as
-the maximum APIC ID.
+uses the return value of KVM_CHECK_EXTENSION(KVM_CAP_MAX_VCPU_ID) - 1
+as the maximum APIC ID.
 
 7.33 KVM_CAP_X86_NOTIFY_VMEXIT
 ------------------------------
-- 
2.55.0.508.g3f0d502094-goog