From: Stefano Garzarella <sgarz...@redhat.com>
Add driver for the vTPM defined by the AMD SVSM spec [1].
The specification defines a protocol that a SEV-SNP guest OS can use to
discover and talk to a vTPM emulated by the Secure VM Service Module (SVSM)
in the guest context, but at a more privileged level (VMPL0).
The new tpm-svsm platform driver uses two functions exposed by x86/sev
to verify that the device is actually emulated by the platform and to
send commands and receive responses.
The device cannot be hot-plugged/unplugged as it is emulated by the
platform, so we can use module_platform_driver_probe(). The probe
function will only check whether in the current runtime configuration,
SVSM is present and provides a vTPM.
This device does not support interrupts and sends responses to commands
synchronously. In order to have .recv() called just after .send() in
tpm_try_transmit(), the .status() callback returns 0, and both
.req_complete_mask and .req_complete_val are set to 0.
[1] "Secure VM Service Module for SEV-SNP Guests"
Publication # 58019 Revision: 1.00
Signed-off-by: Stefano Garzarella <sgarz...@redhat.com>
---
v5:
- removed cancel/status/req_* ops after rebase on master that cotains
commit 980a573621ea ("tpm: Make chip->{status,cancel,req_canceled} opt")
v4:
- moved "asm" includes after the "linux" includes [Tom]
- allocated buffer separately [Tom/Jarkko/Jason]
v3:
- removed send_recv() ops and followed the ftpm driver implementing .status,
.req_complete_mask, .req_complete_val, etc. [Jarkko]
- removed link to the spec because those URLs are unstable [Borislav]
---
drivers/char/tpm/tpm_svsm.c | 135 ++++++++++++++++++++++++++++++++++++
drivers/char/tpm/Kconfig | 10 +++
drivers/char/tpm/Makefile | 1 +
3 files changed, 146 insertions(+)
create mode 100644 drivers/char/tpm/tpm_svsm.c
diff --git a/drivers/char/tpm/tpm_svsm.c b/drivers/char/tpm/tpm_svsm.c
new file mode 100644
index 000000000000..04c532421ff2
--- /dev/null
+++ b/drivers/char/tpm/tpm_svsm.c
@@ -0,0 +1,135 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2025 Red Hat, Inc. All Rights Reserved.
+ *
+ * Driver for the vTPM defined by the AMD SVSM spec [1].
+ *
+ * The specification defines a protocol that a SEV-SNP guest OS can use to
+ * discover and talk to a vTPM emulated by the Secure VM Service Module (SVSM)
+ * in the guest context, but at a more privileged level (usually VMPL0).
+ *
+ * [1] "Secure VM Service Module for SEV-SNP Guests"
+ * Publication # 58019 Revision: 1.00
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/platform_device.h>
+#include <linux/tpm_svsm.h>
+
+#include <asm/sev.h>
+
+#include "tpm.h"
+
+struct tpm_svsm_priv {
+ void *buffer;
+ u8 locality;
+};
+
+static int tpm_svsm_send(struct tpm_chip *chip, u8 *buf, size_t len)
+{
+ struct tpm_svsm_priv *priv = dev_get_drvdata(&chip->dev);
+ int ret;
+
+ ret = svsm_vtpm_cmd_request_fill(priv->buffer, priv->locality, buf,
len);
+ if (ret)
+ return ret;
+
+ /*
+ * The SVSM call uses the same buffer for the command and for the
+ * response, so after this call, the buffer will contain the response
+ * that can be used by .recv() op.
+ */
+ return snp_svsm_vtpm_send_command(priv->buffer);
+}
+
+static int tpm_svsm_recv(struct tpm_chip *chip, u8 *buf, size_t len)
+{
+ struct tpm_svsm_priv *priv = dev_get_drvdata(&chip->dev);
+
+ /*
+ * The internal buffer contains the response after we send the command
+ * to SVSM.
+ */
+ return svsm_vtpm_cmd_response_parse(priv->buffer, buf, len);
+}
+
+static struct tpm_class_ops tpm_chip_ops = {
+ .flags = TPM_OPS_AUTO_STARTUP,
+ .recv = tpm_svsm_recv,
+ .send = tpm_svsm_send,
+};
+
+static int __init tpm_svsm_probe(struct platform_device *pdev)
+{
+ struct device *dev = &pdev->dev;
+ struct tpm_svsm_priv *priv;
+ struct tpm_chip *chip;
+ int err;
+
+ if (!snp_svsm_vtpm_probe())
+ return -ENODEV;
+
+ priv = devm_kmalloc(dev, sizeof(*priv), GFP_KERNEL);
+ if (!priv)
+ return -ENOMEM;
+
+ /*
+ * The maximum buffer supported is one page (see SVSM_VTPM_MAX_BUFFER
+ * in tpm_svsm.h).
+ */
+ priv->buffer = (void *)devm_get_free_pages(dev, GFP_KERNEL, 0);
+ if (!priv->buffer)
+ return -ENOMEM;
+
+ /*
+ * FIXME: before implementing locality we need to agree what it means
+ * for the SNP SVSM vTPM
+ */
+ priv->locality = 0;
