[cpuid] Allow reading hypervisor CPUID leaves

Hypervisors typically intercept CPUID leaves in the range 0x40000000
to 0x400000ff, with leaf 0x40000000 returning the maximum supported
function within this range in register %eax.

iPXE currently masks off bit 30 from the requested CPUID leaf when
checking to see if a function is supported, which causes this check to
read from leaf 0x00000000 instead of 0x40000000.

Fix by including bit 30 within the mask.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/core/cpuid.c

@@ -84,8 +84,8 @@ int cpuid_supported ( uint32_t function ) {
 		return rc;
 
 	/* Find highest supported function number within this family */
-	cpuid ( ( function & CPUID_EXTENDED ), 0, &max_function, &discard_b,
-		&discard_c, &discard_d );
+	cpuid ( ( function & ( CPUID_EXTENDED | CPUID_HYPERVISOR ) ), 0,
+		  &max_function, &discard_b, &discard_c, &discard_d );
 
 	/* Fail if maximum function number is meaningless (e.g. if we
 	 * are attempting to call an extended function on a CPU which

src/arch/x86/include/ipxe/cpuid.h

@@ -33,6 +33,9 @@ struct x86_features {
 /** CPUID extended function */
 #define CPUID_EXTENDED 0x80000000UL
 
+/** CPUID hypervisor function */
+#define CPUID_HYPERVISOR 0x40000000UL
+
 /** Get vendor ID and largest standard function */
 #define CPUID_VENDOR_ID 0x00000000UL