[PATCH] i386: Document when features can be added to kvm_default_props

[Eduardo Habkost <ehabkost@redhat.com>]

It's very easy to mistakenly extend kvm_default_props to include
features that require a kernel version that's too recent.  Add a
comment warning about that, pointing to the documentation file
where the minimum kernel version for KVM is documented.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 target/i386/cpu.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 3ffd877dd51..c8558bb49ac 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -4098,8 +4098,14 @@ static X86CPUDefinition builtin_x86_defs[] = {
     },
 };
 
-/* KVM-specific features that are automatically added/removed
+/*
+ * KVM-specific features that are automatically added/removed
  * from all CPU models when KVM is enabled.
+ *
+ * NOTE: features can be enabled by default only if they were
+ *       already available in the oldest kernel version supported
+ *       by the KVM accelerator (see "OS requirements" section at
+ *       docs/system/target-i386.rst)
  */
 static PropValue kvm_default_props[] = {
     { "kvmclock", "on" },
-- 
2.26.2