[RFC PATCH 6/7] target/i386: SEV: Add support for setting TSC frequency for Secure TSC

Naveen N Rao (AMD)
2025-09-11T11:57:51Z

Add support for configuring the TSC frequency when Secure TSC is enabled
in SEV-SNP guests through a new "tsc-frequency" property on SEV-SNP
guest objects, similar to the vCPU-specific property used by regular
guests and TDX. A new property is needed since SEV-SNP guests require
the TSC frequency to be specified during early SNP_LAUNCH_START command
before any vCPUs are created.

The user-provided TSC frequency is set through KVM_SET_TSC_KHZ before
issuing KVM_SEV_SNP_LAUNCH_START.

Co-developed-by: Ketan Chaturvedi <Ketan.Chaturvedi@amd.com>
Signed-off-by: Ketan Chaturvedi <Ketan.Chaturvedi@amd.com>
Co-developed-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
---
target/i386/sev.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
qapi/qom.json | 6 +++++-
2 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/target/i386/sev.c b/target/i386/sev.c
index 8f88df19a408..facf51c810d9 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -178,6 +178,7 @@ struct SevSnpGuestState {
char *id_auth_base64;
uint8_t *id_auth;
char *host_data;
+ uint32_t tsc_khz;

struct kvm_sev_snp_launch_start kvm_start_conf;
struct kvm_sev_snp_launch_finish kvm_finish_conf;
@@ -536,6 +537,13 @@ static int check_sev_features(SevCommonState *sev_common, uint64_t sev_features,
__func__, sev_features, sev_common->supported_sev_features);
return -1;
}
+ if (sev_snp_enabled() && SEV_SNP_GUEST(sev_common)->tsc_khz &&
+ !(sev_features & SVM_SEV_FEAT_SECURE_TSC)) {
+ error_setg(errp,
+ "%s: TSC frequency can only be set if Secure TSC is enabled",
+ __func__);
+ return -1;
+ }
return 0;
}

@@ -1085,6 +1093,18 @@ sev_snp_launch_start(SevCommonState *sev_common)
return 1;
}

+ if (is_sev_feature_set(sev_common, SVM_SEV_FEAT_SECURE_TSC)) {
+ rc = -EINVAL;
+ if (kvm_check_extension(kvm_state, KVM_CAP_VM_TSC_CONTROL)) {
+ rc = kvm_vm_ioctl(kvm_state, KVM_SET_TSC_KHZ, sev_snp_guest->tsc_khz);
+ }
+ if (rc < 0) {
+ error_report("%s: Unable to set Secure TSC frequency to %u kHz ret=%d",
+ __func__, sev_snp_guest->tsc_khz, rc);
+ return 1;
+ }
+ }
+
rc = sev_ioctl(sev_common->sev_fd, KVM_SEV_SNP_LAUNCH_START,
start, &fw_error);
if (rc < 0) {
@@ -3127,6 +3147,28 @@ static void sev_snp_guest_set_secure_tsc(Object *obj, bool value, Error **errp)
sev_set_feature(SEV_COMMON(obj), SVM_SEV_FEAT_SECURE_TSC, value);
}

+static void
+sev_snp_guest_get_tsc_frequency(Object *obj, Visitor *v, const char *name,
+ void *opaque, Error **errp)
+{
+ uint32_t value = SEV_SNP_GUEST(obj)->tsc_khz * 1000;
+
+ visit_type_uint32(v, name, &value, errp);
+}
+
+static void
+sev_snp_guest_set_tsc_frequency(Object *obj, Visitor *v, const char *name,
+ void *opaque, Error **errp)
+{
+ uint32_t value;
+
+ if (!visit_type_uint32(v, name, &value, errp)) {
+ return;
+ }
+
+ SEV_SNP_GUEST(obj)->tsc_khz = value / 1000;
+}
+
static void
sev_snp_guest_class_init(ObjectClass *oc, const void *data)
{
@@ -3165,6 +3207,9 @@ sev_snp_guest_class_init(ObjectClass *oc, const void *data)
object_class_property_add_bool(oc, "secure-tsc",
sev_snp_guest_get_secure_tsc,
sev_snp_guest_set_secure_tsc);
+ object_class_property_add(oc, "tsc-frequency", "uint32",
+ sev_snp_guest_get_tsc_frequency,
+ sev_snp_guest_set_tsc_frequency, NULL, NULL);
}

static void
diff --git a/qapi/qom.json b/qapi/qom.json
index b05a475ef499..5b99148cb790 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -1102,6 +1102,9 @@
#
# @secure-tsc: enable Secure TSC (default: false) (since 10.2)
#
+# @tsc-frequency: set secure TSC frequency. Only valid if Secure TSC
+# is enabled (default: zero) (since 10.2)
+#
# Since: 9.1
##
{ 'struct': 'SevSnpGuestProperties',
@@ -1114,7 +1117,8 @@
'*author-key-enabled': 'bool',
'*host-data': 'str',
'*vcek-disabled': 'bool',
- '*secure-tsc': 'bool' } }
+ '*secure-tsc': 'bool',
+ '*tsc-frequency': 'uint32' } }

##
# @TdxGuestProperties:
--
2.50.1