image-index: define platform.variant for arm

Signed-off-by: Akihiro Suda <[email protected]>

Author: AkihiroSuda

image-index.md

@@ -63,7 +63,9 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
 
     - **`variant`** *string*
 
-        This OPTIONAL property specifies the variant of the CPU, for example `armv6l` to specify a particular CPU variant of the ARM CPU.
+        This OPTIONAL property specifies the variant of the CPU.
+        When `architecture` is `arm`, values for arch SHOULD use, and runtimes SHOULD understand, arch entries listed in the Go Language document for [`$GOARM`](go-arm). i.e. `5`, `6`, or `7`.
+        When `architecture` is not `arm`, values are implementation-defined and SHOULD be submitted to this specification for standardization.
 
     - **`features`** *array of strings*
 
@@ -114,3 +116,4 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
 
 [runtime-platform2]: https://github.com/opencontainers/runtime-spec/blob/v1.0.0-rc3/config.md#platform
 [matrix]: media-types.md#compatibility-matrix
+[go-arm]: https://golang.org/doc/install/source#environment

schema/backwards_compatibility_test.go

@@ -49,7 +49,7 @@ func TestBackwardsCompatibilityImageIndex(t *testing.T) {
 		fail       bool
 	}{
 		{
-			digest: "sha256:219f4b61132fe9d09b0ec5c15517be2ca712e4744b0e0cc3be71295b35b2a467",
+			digest: "sha256:8c4a37540ca5f211c85c8054c06a432693ff4e53f9fa64c80371a7622a9d4864",
 			imageIndex: `{
    "schemaVersion": 2,
    "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
@@ -91,7 +91,7 @@ func TestBackwardsCompatibilityImageIndex(t *testing.T) {
          "platform": {
             "architecture": "arm",
             "os": "linux",
-            "variant": "armv7"
+            "variant": "7"
          }
       },
       {
@@ -100,8 +100,7 @@ func TestBackwardsCompatibilityImageIndex(t *testing.T) {
          "digest": "sha256:fb2fc0707b86dafa9959fe3d29e66af8787aee4d9a23581714be65db4265ad8a",
          "platform": {
             "architecture": "arm64",
-            "os": "linux",
-            "variant": "armv8"
+            "os": "linux"
          }
       }
    ]

specs-go/v1/descriptor.go

@@ -59,7 +59,7 @@ type Platform struct {
 	OSFeatures []string `json:"os.features,omitempty"`
 
 	// Variant is an optional field specifying a variant of the CPU, for
-	// example `ppc64le` to specify a little-endian version of a PowerPC CPU.
+	// example `7` to specify ARMv7 when architecture is `arm`.
 	Variant string `json:"variant,omitempty"`
 
 	// Features is an optional field specifying an array of strings, each