[v6,1/4] x86/sev: add SVSM vTPM probe/send_command functions

Commit Message

Stefano Garzarella April 3, 2025, 10:09 a.m. UTC

From: Stefano Garzarella <[e-mail]>

Add two new functions to probe and send commands to the SVSM vTPM.
They leverage the two calls defined by the AMD SVSM specification [1]
for the vTPM protocol: SVSM_VTPM_QUERY and SVSM_VTPM_CMD.

Expose these functions to be used by other modules such as a tpm
driver.

[1] "Secure VM Service Module for SEV-SNP Guests"
    Publication # 58019 Revision: 1.00

Co-developed-by: James Bottomley <[e-mail]>
Signed-off-by: James Bottomley <[e-mail]>
Co-developed-by: Claudio Carvalho <[e-mail]>
Signed-off-by: Claudio Carvalho <[e-mail]>
Reviewed-by: Tom Lendacky <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Stefano Garzarella <[e-mail]>
---
v5:
- added stubs when !CONFIG_AMD_MEM_ENCRYPT [Dionna]
- added Jarkko's R-b
v4:
- added Tom's R-b
- added functions documentation [Jarkko]
- simplified TPM_SEND_COMMAND check [Tom/Jarkko]
v3:
- removed link to the spec because those URLs are unstable [Borislav]
- squashed "x86/sev: add SVSM call macros for the vTPM protocol" patch
  in this one [Borislav]
- slimmed down snp_svsm_vtpm_probe() [Borislav]
- removed features check and any print related [Tom]
---
 arch/x86/include/asm/sev.h |  9 ++++++
 arch/x86/coco/sev/core.c   | 59 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

Patch

diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index ba7999f66abe..ba7a0a327afb 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -384,6 +384,10 @@  struct svsm_call {
 #define SVSM_ATTEST_SERVICES		0
 #define SVSM_ATTEST_SINGLE_SERVICE	1
 
+#define SVSM_VTPM_CALL(x)		((2ULL << 32) | (x))
+#define SVSM_VTPM_QUERY			0
+#define SVSM_VTPM_CMD			1
+
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 
 extern u8 snp_vmpl;
@@ -481,6 +485,9 @@  void snp_msg_free(struct snp_msg_desc *mdesc);
 int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req,
 			   struct snp_guest_request_ioctl *rio);
 
+bool snp_svsm_vtpm_probe(void);
+int snp_svsm_vtpm_send_command(u8 *buffer);
+
 void __init snp_secure_tsc_prepare(void);
 void __init snp_secure_tsc_init(void);
 
@@ -524,6 +531,8 @@  static inline struct snp_msg_desc *snp_msg_alloc(void) { return NULL; }
 static inline void snp_msg_free(struct snp_msg_desc *mdesc) { }
 static inline int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req,
 					 struct snp_guest_request_ioctl *rio) { return -ENODEV; }
+static inline bool snp_svsm_vtpm_probe(void) { return false; }
+static inline int snp_svsm_vtpm_send_command(u8 *buffer) { return -ENODEV; }
 static inline void __init snp_secure_tsc_prepare(void) { }
 static inline void __init snp_secure_tsc_init(void) { }
 
diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index b0c1a7a57497..efb43c9d3d30 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -2625,6 +2625,65 @@  static int snp_issue_guest_request(struct snp_guest_req *req, struct snp_req_dat
 	return ret;
 }
 
+/**
+ * snp_svsm_vtpm_probe() - Probe if SVSM provides a vTPM device
+ *
+ * This function checks that there is SVSM and that it supports at least
+ * TPM_SEND_COMMAND which is the only request we use so far.
+ *
+ * Return: true if the platform provides a vTPM SVSM device, false otherwise.
+ */
+bool snp_svsm_vtpm_probe(void)
+{
+	struct svsm_call call = {};
+
+	/* The vTPM device is available only if a SVSM is present */
+	if (!snp_vmpl)
+		return false;
+
+	call.caa = svsm_get_caa();
+	call.rax = SVSM_VTPM_CALL(SVSM_VTPM_QUERY);
+
+	if (svsm_perform_call_protocol(&call))
+		return false;
+
+	/* Check platform commands contains TPM_SEND_COMMAND - platform command 8 */
+	return call.rcx_out & BIT_ULL(8);
+}
+EXPORT_SYMBOL_GPL(snp_svsm_vtpm_probe);
+
+/**
+ * snp_svsm_vtpm_send_command() - execute a vTPM operation on SVSM
+ * @buffer: A buffer used to both send the command and receive the response.
+ *
+ * This function executes a SVSM_VTPM_CMD call as defined by
+ * "Secure VM Service Module for SEV-SNP Guests" Publication # 58019 Revision: 1.00
+ *
+ * All command request/response buffers have a common structure as specified by
+ * the following table:
+ *     Byte      Size       In/Out    Description
+ *     Offset    (Bytes)
+ *     0x000     4          In        Platform command
+ *                          Out       Platform command response size
+ *
+ * Each command can build upon this common request/response structure to create
+ * a structure specific to the command.
+ * See include/linux/tpm_svsm.h for more details.
+ *
+ * Return: 0 on success, -errno on failure
+ */
+int snp_svsm_vtpm_send_command(u8 *buffer)
+{
+	struct svsm_call call = {};
+
+	call.caa = svsm_get_caa();
+	call.rax = SVSM_VTPM_CALL(SVSM_VTPM_CMD);
+	call.rcx = __pa(buffer);
+
+	return svsm_perform_call_protocol(&call);
+}
+EXPORT_SYMBOL_GPL(snp_svsm_vtpm_send_command);
+
 static struct platform_device sev_guest_device = {
 	.name		= "sev-guest",
 	.id		= -1,