Set fixed field in CPUID function 2

From the SDM: "The least-significant byte in register EAX (register AL)
will always return 01H. Software should ignore this value and not
interpret it as an informational descriptor."

Unfortunately, online docs [1] [2] (likely based on an old version of the SDM)
say: "The least-significant byte in register EAX (register AL) indicates
the number of times the CPUID instruction must be executed with an input
value of 2 to get a complete description of the processor's caches and
TLBs."

dlang uses this second interpretation [3] and will loop 2^32 times if we
return zero. Fix this by specifying the fixed value of one. We still
don't support exposing the actual cache information, leaving all other
bytes empty. A zero byte means: "Null descriptor, this byte contains no
information."

[1] http://www.sandpile.org/x86/cpuid.htm#level_0000_0002h
[2] https://c9x.me/x86/html/file_module_x86_id_45.html
[3] https://github.com/dlang/druntime/blob/424640864c2aa001731467e96f637bd3e704e481/src/core/cpuid.d#L533-L534

PiperOrigin-RevId: 242046629
Change-Id: Ic0f0a5f974b20f71391cb85645bdcd4003e5fe88

1 files changed, 25 insertions, 1 deletions

diff --git a/pkg/cpuid/cpuid.go b/pkg/cpuid/cpuid.go
index 0571b7cde..64e2e68f1 100644
--- a/pkg/cpuid/cpuid.go
+++ b/pkg/cpuid/cpuid.go
@@ -551,6 +551,16 @@ func (fs FeatureSet) CPUInfo(cpu uint) string {
 	return b.String()
 }
 
+// AMD returns true if fs describes an AMD CPU.
+func (fs *FeatureSet) AMD() bool {
+	return fs.VendorID == "AuthenticAMD"
+}
+
+// Intel returns true if fs describes an Intel CPU.
+func (fs *FeatureSet) Intel() bool {
+	return fs.VendorID == "GenuineIntel"
+}
+
 // Helper to convert 3 regs into 12-byte vendor ID.
 func vendorIDFromRegs(bx, cx, dx uint32) string {
 	bytes := make([]byte, 0, 12)
@@ -735,6 +745,20 @@ func (fs *FeatureSet) EmulateID(origAx, origCx uint32) (ax, bx, cx, dx uint32) {
 		cx = fs.blockMask(block(0))
 		dx = fs.blockMask(block(1))
 		ax = fs.signature()
+	case intelCacheDescriptors:
+		if !fs.Intel() {
+			// Reserved on non-Intel.
+			return 0, 0, 0, 0
+		}
+
+		// "The least-significant byte in register EAX (register AL)
+		// will always return 01H. Software should ignore this value
+		// and not interpret it as an informational descriptor." - SDM
+		//
+		// We do not support exposing cache information, but we do set
+		// this fixed field because some language runtimes (dlang) get
+		// confused by ax = 0 and will loop infinitely.
+		ax = 1
 	case xSaveInfo:
 		if !fs.UseXsave() {
 			return 0, 0, 0, 0
@@ -753,7 +777,7 @@ func (fs *FeatureSet) EmulateID(origAx, origCx uint32) (ax, bx, cx, dx uint32) {
 	case extendedFeatures:
 		cx = fs.blockMask(block(5))
 		dx = fs.blockMask(block(6))
-		if fs.VendorID == "AuthenticAMD" {
+		if fs.AMD() {
 			// AMD duplicates some block 1 features in block 6.
 			dx |= fs.blockMask(block(1)) & block6DuplicateMask
 		}