[hyperv] Do not steal ownership from the Gen 2 UEFI firmware

We must not steal ownership from the Gen 2 UEFI firmware, since doing
so will cause an immediate system crash (most likely in the form of a
reboot).

This problem was masked before commit a0f6e75 ("[hyperv] Do not fail
if guest OS ID MSR is already set"), since prior to that commit we
would always fail if we found any non-zero guest OS identity.  We now
accept a non-zero previous guest OS identity in order to allow for
situations such as chainloading from iPXE to another iPXE, and as a
prerequisite for commit b91cc98 ("[hyperv] Cope with Windows Server
2016 enlightenments").

A proper fix would be to reverse engineer the UEFI protocols exposed
within the Hyper-V Gen 2 firmware and use these to bind to the VMBus
device representing the network connection, (with the native Hyper-V
driver moved to become a BIOS-only feature).

As an interim solution, fail to initialise the native Hyper-V driver
if we detect the guest OS identity known to be used by the Gen 2 UEFI
firmware.  This will cause the standard all-drivers build (ipxe.efi)
to fall back to using the SNP driver.

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

src/arch/x86/drivers/hyperv/hyperv.c

 	return 0;
 }
 
+/**
+ * Check that Gen 2 UEFI firmware is not running
+ *
+ * @v hv		Hyper-V hypervisor
+ * @ret rc		Return status code
+ *
+ * We must not steal ownership from the Gen 2 UEFI firmware, since
+ * doing so will cause an immediate crash.  Avoid this by checking for
+ * the guest OS identity known to be used by the Gen 2 UEFI firmware.
+ */
+static int hv_check_uefi ( struct hv_hypervisor *hv ) {
+	uint64_t guest_os_id;
+
+	/* Check for UEFI firmware's guest OS identity */
+	guest_os_id = rdmsr ( HV_X64_MSR_GUEST_OS_ID );
+	if ( guest_os_id == HV_GUEST_OS_ID_UEFI ) {
+		DBGC ( hv, "HV %p is owned by UEFI firmware\n", hv );
+		return -ENOTSUP;
+	}
+
+	return 0;
+}
+
 /**
  * Map hypercall page
  *
 	if ( ( rc = hv_check_features ( hv ) ) != 0 )
 		goto err_check_features;
 
+	/* Check that Gen 2 UEFI firmware is not running */
+	if ( ( rc = hv_check_uefi ( hv ) ) != 0 )
+		goto err_check_uefi;
+
 	/* Allocate pages */
 	if ( ( rc = hv_alloc_pages ( hv, &hv->hypercall, &hv->synic.message,
 				     &hv->synic.event, NULL ) ) != 0 )
 	hv_free_pages ( hv, hv->hypercall, hv->synic.message, hv->synic.event,
 			NULL );
  err_alloc_pages:
+ err_check_uefi:
  err_check_features:
 	free ( hv );
  err_alloc:

src/include/ipxe/hyperv.h

  */
 #define HV_GUEST_OS_ID_IPXE ( ( 1ULL << 63 ) | ( 0x18aeULL << 48 ) )
 
+/** Guest OS identity for Gen 2 UEFI firmware
+ *
+ * This does not conform to the documented structure for guest OS
+ * identities.
+ */
+#define HV_GUEST_OS_ID_UEFI ( 1ULL << 40 )
+
 /** Enable hypercall page */
 #define HV_HYPERCALL_ENABLE 0x00000001UL