Re: [PATCH v3 1/4] x86/sev: add SVSM vTPM probe/send_command functions
From: Stefano Garzarella
Date: Wed Mar 12 2025 - 06:56:29 EST
On Tue, Mar 11, 2025 at 11:56:23AM +0200, Jarkko Sakkinen wrote:
On Tue, Mar 11, 2025 at 10:42:22AM +0100, Stefano Garzarella wrote:
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 <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
Co-developed-by: Claudio Carvalho <cclaudio@xxxxxxxxxxxxx>
Signed-off-by: Claudio Carvalho <cclaudio@xxxxxxxxxxxxx>
Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
---
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 | 7 +++++++
arch/x86/coco/sev/core.c | 31 +++++++++++++++++++++++++++++++
2 files changed, 38 insertions(+)
diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index ba7999f66abe..09471d058ce5 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);
diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 96c7bc698e6b..2166bdff88b7 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -2628,6 +2628,37 @@ static int snp_issue_guest_request(struct snp_guest_req *req, struct snp_req_dat
return ret;
}
Since this is an exported symbol, it'd be a good practice document
snp_svsm_vtpm_probe().
Yes, you are right, since the others were not documented, I had not
added it, but I agree with you, I'll do in v4.
+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);
I supposed CAA is some kind of shared memory area for host and VM?
Not with the host, but with SVSM, which is the firmware running in the
guest, but at a higher privilege level (VMPL) than the kernel, where,
for example, the vTPM is emulated.
BTW, yep it is a shared memory defined by the SVSM calling convention.
From AMD SVSM specification:
5 Calling Convention
Each call to the SVSM conveys data through a combination of the
SVSM Calling Area (whose address was first configured through the
SVSM_CAA field of the secrets page) and registers. Use of the
Calling Area is necessary for the SVSM to detect the difference
between a call that was issued by the guest and a spurious
invocation by a poorly behaved host. Registers are used for all
other parameters.
The initially configured SVSM Calling Area is a page of memory that
lies outside the initial SVSM memory range and has not had its VMPL
permissions restricted in any way. The address is guaranteed to be
aligned to a 4 KB boundary, so the remainder of the page may be used
by the guest for memory-based parameter passing if desired.
The contents of the Calling Area are described in the following
table:
Table 2: Calling Area
Byte Size Name Description
Offset
0x000 1 byte SVSM_CALL_PENDING Indicates whether a call has
been requested by the guest
(0=no call requested, 1=call
requested).
0x001 1 byte SVSM_MEM_AVAILABLE Free memory is available to
be withdrawn.
0x002 6 byte Reserved. The SVSM is not
required to verify that
these bytes are 0.
+
+ 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)) == BIT_ULL(8);
+}
+EXPORT_SYMBOL_GPL(snp_svsm_vtpm_probe);
+
Ditto.
Ack.
+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,
--
2.48.1
That said, these are rather self-documenting (i.e, nice and clean).
Thanks for the review!
Stefano