Software needs to talk with P-SEAMLDR via P-SEAMLDR SEAMCALLs. So, add a
wrapper for P-SEAMLDR SEAMCALLs.

Save and restore the current VMCS using VMPTRST and VMPTRLD instructions
to avoid breaking KVM. Doing so is because P-SEAMLDR SEAMCALLs would
invalidate the current VMCS as documented in Intel® Trust Domain CPU
Architectural Extensions (May 2021 edition) Chapter 2.3 [1]:

  SEAMRET from the P-SEAMLDR clears the current VMCS structure pointed
  to by the current-VMCS pointer. A VMM that invokes the P-SEAMLDR using
  SEAMCALL must reload the current-VMCS, if required, using the VMPTRLD
  instruction.

Disable interrupts to prevent KVM code from interfering with P-SEAMLDR
SEAMCALLs. For example, if a vCPU is scheduled before the current VMCS is
restored, it may encounter an invalid current VMCS, causing its VMX
instruction to fail. Additionally, if KVM sends IPIs to invalidate a
current VMCS and the invalidation occurs right after the current VMCS is
saved, that VMCS will be reloaded after P-SEAMLDR SEAMCALLs, leading to
unexpected behavior.

NMIs are not a problem, as the only scenario where instructions relying on
the current-VMCS are used is during guest PMI handling in KVM. This occurs
immediately after VM exits with IRQ and NMI disabled, ensuring no
interference with P-SEAMLDR SEAMCALLs.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Tested-by: Farrah Chen <farrah.chen@intel.com>
Link: https://cdrdv2.intel.com/v1/dl/getContent/733582 # [1]
---
v2:
 - don't create a new, inferior framework to save/restore VMCS
 - use human-friendly language, just "current VMCS" rather than
   SDM term "current-VMCS pointer"
 - don't mix guard() with goto
---
 arch/x86/virt/vmx/tdx/Makefile     |  1 +
 arch/x86/virt/vmx/tdx/seamldr.c    | 56 ++++++++++++++++++++++++++++++
 drivers/virt/coco/tdx-host/Kconfig | 10 ++++++
 3 files changed, 67 insertions(+)
 create mode 100644 arch/x86/virt/vmx/tdx/seamldr.c

diff --git a/arch/x86/virt/vmx/tdx/Makefile b/arch/x86/virt/vmx/tdx/Makefile
index 90da47eb85ee..26aea3531c36 100644
--- a/arch/x86/virt/vmx/tdx/Makefile
+++ b/arch/x86/virt/vmx/tdx/Makefile
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0-only
 obj-y += seamcall.o tdx.o
+obj-$(CONFIG_INTEL_TDX_MODULE_UPDATE) += seamldr.o
diff --git a/arch/x86/virt/vmx/tdx/seamldr.c b/arch/x86/virt/vmx/tdx/seamldr.c
new file mode 100644
index 000000000000..b99d73f7bb08
--- /dev/null
+++ b/arch/x86/virt/vmx/tdx/seamldr.c
@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright(c) 2025 Intel Corporation.
+ *
+ * Intel TDX module runtime update
+ */
+#define pr_fmt(fmt)	"seamldr: " fmt
+
+#include <linux/irqflags.h>
+#include <linux/types.h>
+
+#include "seamcall.h"
+
+static __maybe_unused int seamldr_call(u64 fn, struct tdx_module_args *args)
+{
+	unsigned long flags;
+	u64 vmcs;
+	int ret;
+
+	if (!is_seamldr_call(fn))
+		return -EINVAL;
+
+	/*
+	 * SEAMRET from P-SEAMLDR invalidates the current VMCS.  Save/restore
+	 * the VMCS across P-SEAMLDR SEAMCALLs to avoid clobbering KVM state.
+	 * Disable interrupts as KVM is allowed to do VMREAD/VMWRITE in IRQ
+	 * context (but not NMI context).
+	 */
+	local_irq_save(flags);
+
+	asm goto("1: vmptrst %0\n\t"
+		 _ASM_EXTABLE(1b, %l[error])
+		 : "=m" (vmcs) : : "cc" : error);
+
+	ret = seamldr_prerr(fn, args);
+
+	/*
+	 * Restore the current VMCS pointer.  VMPTSTR "returns" all ones if the
+	 * current VMCS is invalid.
+	 */
+	if (vmcs != -1ULL) {
+		asm goto("1: vmptrld %0\n\t"
+			 "jna %l[error]\n\t"
+			 _ASM_EXTABLE(1b, %l[error])
+			 : : "m" (vmcs) : "cc" : error);
+	}
+
+	local_irq_restore(flags);
+	return ret;
+
+error:
+	local_irq_restore(flags);
+
+	WARN_ONCE(1, "Failed to save/restore the current VMCS");
+	return -EIO;
+}
diff --git a/drivers/virt/coco/tdx-host/Kconfig b/drivers/virt/coco/tdx-host/Kconfig
index e58bad148a35..6a9199e6c2c6 100644
--- a/drivers/virt/coco/tdx-host/Kconfig
+++ b/drivers/virt/coco/tdx-host/Kconfig
@@ -8,3 +8,13 @@ config TDX_HOST_SERVICES
 
 	  Say y or m if enabling support for confidential virtual machine
 	  support (CONFIG_INTEL_TDX_HOST). The module is called tdx_host.ko
+
+config INTEL_TDX_MODULE_UPDATE
+	bool "Intel TDX module runtime update"
+	depends on TDX_HOST_SERVICES
+	help
+	  This enables the kernel to support TDX module runtime update. This
+	  allows the admin to update the TDX module to another compatible
+	  version without the need to terminate running TDX guests.
+
+	  If unsure, say N.
-- 
2.47.3