diff --git a/image-index.md b/image-index.md
index c85fd47b8..8ee6d406c 100644
--- a/image-index.md
+++ b/image-index.md
@@ -67,7 +67,13 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
 
     - **`features`** *array of strings*
 
-        This OPTIONAL property specifies an array of strings, each specifying a mandatory CPU feature (for example `sse4` or `aes`).
+        This OPTIONAL property specifies an array of strings, each specifying a mandatory CPU feature.
+
+        When `architecture` is `386` or `amd64`, image indexes SHOULD use, and implementations SHOULD understand, values [supported by Linux][cpufeatures.h] with the `X86_FEATURE_` prefix removed and the remainder lowercased.
+        For example, include `vmx` (for `X86_FEATURE_VMX`) if the image contains an executable compiled to use `VMXON` and related instructions with no fallback.
+        On Linux on these architectures, the features supported by host CPUs can be found in the `flags` entries in the `cpuinfo` file provided by the [proc filesystem][proc.5].
+
+        When `architecture` is neither `386` nor `amd64`, values are implementation-defined and SHOULD be submitted to this specification for standardization.
 
 - **`annotations`** *string-string map*
 
@@ -99,8 +105,8 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
       "platform": {
         "architecture": "amd64",
         "os": "linux",
-        "os.features": [
-          "sse4"
+        "features": [
+          "fpu"
         ]
       }
     }
@@ -112,5 +118,7 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
 }
 ```
 
+[cpufeatures.h]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/include/asm/cpufeatures.h
+[proc.5]: http://man7.org/linux/man-pages/man5/proc.5.html
 [runtime-platform2]: /~https://github.com/opencontainers/runtime-spec/blob/v1.0.0-rc3/config.md#platform
 [matrix]: media-types.md#compatibility-matrix
diff --git a/schema/imageindex_test.go b/schema/imageindex_test.go
index 69d1a1fa0..37d33255e 100644
--- a/schema/imageindex_test.go
+++ b/schema/imageindex_test.go
@@ -61,7 +61,7 @@ func TestImageIndex(t *testing.T) {
         "architecture": "amd64",
         "os": "linux",
         "features": [
-          "sse4"
+          "vmx"
         ]
       }
     }
@@ -84,7 +84,7 @@ func TestImageIndex(t *testing.T) {
         "architecture": "amd64",
         "os": "linux",
         "features": [
-          "sse4"
+          "vmx"
         ]
       }
     }
@@ -128,7 +128,7 @@ func TestImageIndex(t *testing.T) {
         "architecture": "amd64",
         "os": "linux",
         "features": [
-          "sse4"
+          "vmx"
         ]
       }
     }
@@ -152,7 +152,7 @@ func TestImageIndex(t *testing.T) {
         "architecture": "amd64",
         "os": "linux",
         "features": [
-          "sse4"
+          "vmx"
         ]
       }
     }
@@ -185,7 +185,7 @@ func TestImageIndex(t *testing.T) {
         "architecture": "amd64",
         "os": "linux",
         "features": [
-          "sse4"
+          "vmx"
         ]
       }
     }
diff --git a/specs-go/v1/index.go b/specs-go/v1/index.go
index c7d56ef66..0c8a35828 100644
--- a/specs-go/v1/index.go
+++ b/specs-go/v1/index.go
@@ -38,7 +38,7 @@ type Platform struct {
 	Variant string `json:"variant,omitempty"`
 
 	// Features is an optional field specifying an array of strings, each
-	// listing a required CPU feature (for example `sse4` or `aes`).
+	// listing a required CPU feature.
 	Features []string `json:"features,omitempty"`
 }