[PATCH v4 02/24] coco/tdx-host: Introduce a "tdx_host" device

[Chao Gao <chao.gao@intel.com>]

TDX depends on a platform firmware module that is invoked via instructions
similar to vmenter (i.e. enter into a new privileged "root-mode" context to
manage private memory and private device mechanisms). It is a software
construct that depends on the CPU vmxon state to enable invocation of
TDX-module ABIs. Unlike other Trusted Execution Environment (TEE) platform
implementations that employ a firmware module running on a PCI device with
an MMIO mailbox for communication, TDX has no hardware device to point to
as the TEE Secure Manager (TSM).

Create a virtual device not only to align with other implementations but
also to make it easier to

 - expose metadata (e.g., TDX module version, seamldr version etc) to
   the userspace as device attributes

 - implement firmware uploader APIs which are tied to a device. This is
   needed to support TDX module runtime updates

 - enable TDX Connect which will share a common infrastructure with other
   platform implementations. In the TDX Connect context, every
   architecture has a TSM, represented by a PCIe or virtual device. The
   new "tdx_host" device will serve the TSM role.

A faux device is used as for TDX because the TDX module is singular within
the system and lacks associated platform resources. Using a faux device
eliminates the need to create a stub bus.

The call to tdx_get_sysinfo() ensures that the TDX Module is ready to
provide services.

Note that AMD has a PCI device for the PSP for SEV and ARM CCA will
likely have a faux device [1].

Co-developed-by: Xu Yilun <yilun.xu@linux.intel.com>
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Reviewed-by: Xu Yilun <yilun.xu@linux.intel.com>
Link: https://lore.kernel.org/all/2025073035-bulginess-rematch-b92e@gregkh/ # [1]
---
v3:
 - add Jonathan's reviewed-by
 - add tdx_get_sysinfo() in module_init() to ensure the TDX Module is up
   and running.
 - note in the changelog that both AMD and ARM have devices for coco
---
 arch/x86/virt/vmx/tdx/tdx.c           |  2 +-
 drivers/virt/coco/Kconfig             |  2 ++
 drivers/virt/coco/Makefile            |  1 +
 drivers/virt/coco/tdx-host/Kconfig    | 10 +++++++
 drivers/virt/coco/tdx-host/Makefile   |  1 +
 drivers/virt/coco/tdx-host/tdx-host.c | 43 +++++++++++++++++++++++++++
 6 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 drivers/virt/coco/tdx-host/Kconfig
 create mode 100644 drivers/virt/coco/tdx-host/Makefile
 create mode 100644 drivers/virt/coco/tdx-host/tdx-host.c

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index ddcc1a8c743f..b65b2a609e81 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -1435,7 +1435,7 @@ const struct tdx_sys_info *tdx_get_sysinfo(void)
 
 	return p;
 }
-EXPORT_SYMBOL_FOR_KVM(tdx_get_sysinfo);
+EXPORT_SYMBOL_FOR_MODULES(tdx_get_sysinfo, "kvm-intel,tdx-host");
 
 u32 tdx_get_nr_guest_keyids(void)
 {
diff --git a/drivers/virt/coco/Kconfig b/drivers/virt/coco/Kconfig
index df1cfaf26c65..f7691f64fbe3 100644
--- a/drivers/virt/coco/Kconfig
+++ b/drivers/virt/coco/Kconfig
@@ -17,5 +17,7 @@ source "drivers/virt/coco/arm-cca-guest/Kconfig"
 source "drivers/virt/coco/guest/Kconfig"
 endif
 
+source "drivers/virt/coco/tdx-host/Kconfig"
+
 config TSM
 	bool
diff --git a/drivers/virt/coco/Makefile b/drivers/virt/coco/Makefile
index cb52021912b3..b323b0ae4f82 100644
--- a/drivers/virt/coco/Makefile
+++ b/drivers/virt/coco/Makefile
@@ -6,6 +6,7 @@ obj-$(CONFIG_EFI_SECRET)	+= efi_secret/
 obj-$(CONFIG_ARM_PKVM_GUEST)	+= pkvm-guest/
 obj-$(CONFIG_SEV_GUEST)		+= sev-guest/
 obj-$(CONFIG_INTEL_TDX_GUEST)	+= tdx-guest/
+obj-$(CONFIG_INTEL_TDX_HOST)	+= tdx-host/
 obj-$(CONFIG_ARM_CCA_GUEST)	+= arm-cca-guest/
 obj-$(CONFIG_TSM) 		+= tsm-core.o
 obj-$(CONFIG_TSM_GUEST)		+= guest/
diff --git a/drivers/virt/coco/tdx-host/Kconfig b/drivers/virt/coco/tdx-host/Kconfig
new file mode 100644
index 000000000000..e58bad148a35
--- /dev/null
+++ b/drivers/virt/coco/tdx-host/Kconfig
@@ -0,0 +1,10 @@
+config TDX_HOST_SERVICES
+	tristate "TDX Host Services Driver"
+	depends on INTEL_TDX_HOST
+	default m
+	help
+	  Enable access to TDX host services like module update and
+	  extensions (e.g. TDX Connect).
+
+	  Say y or m if enabling support for confidential virtual machine
+	  support (CONFIG_INTEL_TDX_HOST). The module is called tdx_host.ko
diff --git a/drivers/virt/coco/tdx-host/Makefile b/drivers/virt/coco/tdx-host/Makefile
new file mode 100644
index 000000000000..e61e749a8dff
--- /dev/null
+++ b/drivers/virt/coco/tdx-host/Makefile
@@ -0,0 +1 @@
+obj-$(CONFIG_TDX_HOST_SERVICES) += tdx-host.o
diff --git a/drivers/virt/coco/tdx-host/tdx-host.c b/drivers/virt/coco/tdx-host/tdx-host.c
new file mode 100644
index 000000000000..c77885392b09
--- /dev/null
+++ b/drivers/virt/coco/tdx-host/tdx-host.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * TDX host user interface driver
+ *
+ * Copyright (C) 2025 Intel Corporation
+ */
+
+#include <linux/device/faux.h>
+#include <linux/module.h>
+#include <linux/mod_devicetable.h>
+
+#include <asm/cpu_device_id.h>
+#include <asm/tdx.h>
+
+static const struct x86_cpu_id tdx_host_ids[] = {
+	X86_MATCH_FEATURE(X86_FEATURE_TDX_HOST_PLATFORM, NULL),
+	{}
+};
+MODULE_DEVICE_TABLE(x86cpu, tdx_host_ids);
+
+static struct faux_device *fdev;
+
+static int __init tdx_host_init(void)
+{
+	if (!x86_match_cpu(tdx_host_ids) || !tdx_get_sysinfo())
+		return -ENODEV;
+
+	fdev = faux_device_create(KBUILD_MODNAME, NULL, NULL);
+	if (!fdev)
+		return -ENODEV;
+
+	return 0;
+}
+module_init(tdx_host_init);
+
+static void __exit tdx_host_exit(void)
+{
+	faux_device_destroy(fdev);
+}
+module_exit(tdx_host_exit);
+
+MODULE_DESCRIPTION("TDX Host Services");
+MODULE_LICENSE("GPL");
-- 
2.47.3