2 files changed, 34 insertions, 8 deletions
diff --git a/internal/stupidgcm/cipher_suites.go b/internal/stupidgcm/cipher_suites.go
new file mode 100644
index 0000000..fd032fd
--- /dev/null
+++ b/internal/stupidgcm/cipher_suites.go
@@ -0,0 +1,28 @@
+package stupidgcm
+
+import (
+	"runtime"
+
+	"golang.org/x/sys/cpu"
+)
+
+// ********
+// Carbon-copied from Go Stdlib
+// https://github.com/golang/go/blob/45967bb18e04fa6dc62c2786c87ce120443c64f6/src/crypto/tls/cipher_suites.go#L367
+// ********
+
+var (
+	hasGCMAsmAMD64 = cpu.X86.HasAES && cpu.X86.HasPCLMULQDQ
+	hasGCMAsmARM64 = cpu.ARM64.HasAES && cpu.ARM64.HasPMULL
+	// Keep in sync with crypto/aes/cipher_s390x.go.
+	hasGCMAsmS390X = cpu.S390X.HasAES && cpu.S390X.HasAESCBC && cpu.S390X.HasAESCTR &&
+		(cpu.S390X.HasGHASH || cpu.S390X.HasAESGCM)
+
+	hasAESGCMHardwareSupport = runtime.GOARCH == "amd64" && hasGCMAsmAMD64 ||
+		runtime.GOARCH == "arm64" && hasGCMAsmARM64 ||
+		runtime.GOARCH == "s390x" && hasGCMAsmS390X
+)
+
+// ********
+// End carbon-copy
+// ********
diff --git a/internal/stupidgcm/prefer.go b/internal/stupidgcm/prefer.go
index e3f52d4..6a8cf77 100644
--- a/internal/stupidgcm/prefer.go
+++ b/internal/stupidgcm/prefer.go
@@ -2,8 +2,6 @@ package stupidgcm
 
 import (
 	"runtime"
-
-	"golang.org/x/sys/cpu"
 )
 
 // PreferOpenSSLAES256GCM tells us if OpenSSL AES-256-GCM is faster than Go stdlib
@@ -22,7 +20,7 @@ func PreferOpenSSLAES256GCM() bool {
 		return false
 	}
 	// If the CPU has AES acceleration, Go stdlib is faster
-	if CpuHasAES() {
+	if HasAESGCMHardwareSupport() {
 		return false
 	}
 	// Otherwise OpenSSL is probably faster
@@ -44,13 +42,13 @@ func PreferOpenSSLXchacha20poly1305() bool {
 	return true
 }
 
-// CpuHasAES tells you if the CPU we are running has AES acceleration that is
-// usable by the Go crypto library.
-func CpuHasAES() bool {
-	// Safe to call on other architectures - will just read false.
-	if cpu.X86.HasAES || cpu.ARM64.HasAES {
+// HasAESGCMHardwareSupport tells you if the CPU we are running has AES-GCM
+// acceleration that is usable by the Go crypto library.
+func HasAESGCMHardwareSupport() bool {
+	if hasAESGCMHardwareSupport {
 		return true
 	}
+
 	// On the Apple M1, the CPU has AES acceleration, despite cpu.ARM64.HasAES
 	// reading false: https://github.com/rfjakob/gocryptfs/issues/556#issuecomment-848079309
 	if runtime.GOOS == "darwin" && runtime.GOARCH == "arm64" {