Re: [PATCH 1/4] sev: rename set_guest_policy to set_id_block and remove dead policy code

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, Sep 01, 2026 at 12:09:18PM +0200, Luigi Leonardi wrote:
>The guest policy must be provided at guest launch start: LAUNCH_START for
>SEV/SEV-ES and SNP_LAUNCH_START for SEV-SNP. See the SEV API
>specification, chapter 3 (Guest Policy), and the SEV-SNP firmware ABI
>specification, section 4.3 (Guest Policy).
>
>The policy parameter in set_guest_policy was never effective: by the
>time this callback runs, LAUNCH_START has already been issued for both
>SEV/SEV-ES and SEV-SNP, so writing to kvm_start_conf.policy or
>sev_guest->policy has no effect. In practice the only thing this
>callback actually does is set the ID block and ID auth for SNP's
>LAUNCH_FINISH, so rename it to set_id_block to reflect its real
>purpose, remove the unused policy parameter, and drop the non-SNP code
>path which was entirely dead.
>
>This is preliminary work: actually forwarding the guest policy to the
>platform before LAUNCH_START is added in a later commit.

IIUC the behaviour is the same after this patch, but IMO better to
clarify.

>
>Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
>---
> backends/confidential-guest-support.c       |  12 ++-
> backends/igvm.c                             |   6 +-
> include/system/confidential-guest-support.h |  28 ++++---
> target/i386/sev.c                           | 118 +++++++++++-----------------
> 4 files changed, 67 insertions(+), 97 deletions(-)
>
>diff --git a/backends/confidential-guest-support.c b/backends/confidential-guest-support.c
>index 156dd15e66..a0b36d2da5 100644
>--- a/backends/confidential-guest-support.c
>+++ b/backends/confidential-guest-support.c
>@@ -38,14 +38,12 @@ static int set_guest_state(hwaddr gpa, uint8_t *ptr, uint64_t len,
>     return -1;
> }
>
>-static int set_guest_policy(ConfidentialGuestPolicyType policy_type,
>-                            uint64_t policy,
>-                            void *policy_data1, uint32_t policy_data1_size,
>-                            void *policy_data2, uint32_t policy_data2_size,
>-                            Error **errp)
>+static int set_id_block(void *id_block, uint32_t id_block_size,
>+                        void *id_auth, uint32_t id_auth_size,
>+                        Error **errp)
> {
>     error_setg(errp,
>-               "Setting confidential guest policy is not supported for this platform");
>+               "Setting ID block is not supported for this platform");
>     return -1;
> }
>
>@@ -64,7 +62,7 @@ static void confidential_guest_support_class_init(ObjectClass *oc,
>     ConfidentialGuestSupportClass *cgsc = CONFIDENTIAL_GUEST_SUPPORT_CLASS(oc);
>     cgsc->check_support = check_support;
>     cgsc->set_guest_state = set_guest_state;
>-    cgsc->set_guest_policy = set_guest_policy;
>+    cgsc->set_id_block = set_id_block;
>     cgsc->get_mem_map_entry = get_mem_map_entry;
> }
>
>diff --git a/backends/igvm.c b/backends/igvm.c
>index 7b7bdc72b7..85de0d54ec 100644
>--- a/backends/igvm.c
>+++ b/backends/igvm.c
>@@ -968,9 +968,9 @@ static int qigvm_handle_policy(QIgvm *ctx, Error **errp)
>             id_block_len = sizeof(struct sev_id_block);
>             id_auth_len = sizeof(struct sev_id_authentication);
>         }
>-        return ctx->cgsc->set_guest_policy(GUEST_POLICY_SEV, 
>ctx->sev_policy,
>-                                          ctx->id_block, id_block_len,
>-                                          ctx->id_auth, id_auth_len, errp);
>+
>+        return ctx->cgsc->set_id_block(ctx->id_block, id_block_len,
>+                                       ctx->id_auth, id_auth_len, errp);
>     }
>     return 0;
> }
>diff --git a/include/system/confidential-guest-support.h b/include/system/confidential-guest-support.h
>index 5dca717308..6d35ddb97a 100644
>--- a/include/system/confidential-guest-support.h
>+++ b/include/system/confidential-guest-support.h
>@@ -128,21 +128,23 @@ typedef struct ConfidentialGuestSupportClass {
>                            uint16_t cpu_index, Error **errp);
>
>     /*
>-     * Set the guest policy. The policy can be used to configure the
>-     * confidential platform, such as if debug is enabled or not and can contain
>-     * information about expected launch measurements, signed verification of
>-     * guest configuration and other platform data.
>-     *
>-     * The format of the policy data is specific to each platform. For example,
>-     * SEV-SNP uses a policy bitfield in the 'policy' argument and provides an
>-     * ID block and ID authentication in the 'policy_data' parameters. The type
>-     * of policy data is identified by the 'policy_type' argument.
>+     * Set the guest policy for the confidential platform. The policy
>+     * configures properties of the guest, such as whether debug is
>+     * enabled. Its format is platform-specific; for SEV/SEV-ES and
>+     * SEV-SNP it is a policy bitfield. Must be called before LAUNCH_START
>+     * so the policy is in effect for launch.
>      */
>     int (*set_guest_policy)(ConfidentialGuestPolicyType policy_type,

I'm confused, this commit says "rename set_guest_policy to set_id_block"
so why this callback is still here?

>-                            uint64_t policy,
>-                            void *policy_data1, uint32_t policy_data1_size,
>-                            void *policy_data2, uint32_t policy_data2_size,
>-                            Error **errp);
>+                            uint64_t policy, Error **errp);
>+
>+    /*
>+     * Set the SEV-SNP ID block and ID authentication block. These are
>+     * passed to SNP_LAUNCH_FINISH to provide signed verification of the
>+     * guest configuration.
>+     */
>+    int (*set_id_block)(void *id_block, uint32_t id_block_size,
>+                        void *id_auth, uint32_t id_auth_size,
>+                        Error **errp);
>
>     /*
>      * Iterate the system memory map, getting the entry with the given index
>diff --git a/target/i386/sev.c b/target/i386/sev.c
>index 4d875d10ff..465415c535 100644
>--- a/target/i386/sev.c
>+++ b/target/i386/sev.c
>@@ -2723,10 +2723,9 @@ static int cgs_get_mem_map_entry(int index,
>     return 0;
> }
>
>-static int cgs_set_guest_policy(ConfidentialGuestPolicyType policy_type,
>-                                uint64_t policy, void *policy_data1,
>-                                uint32_t policy_data1_size, void *policy_data2,
>-                                uint32_t policy_data2_size, Error **errp)
>+static int cgs_set_id_block(void *id_block, uint32_t id_block_size,
>+                            void *id_auth, uint32_t id_auth_size,
>+                            Error **errp)
> {
>     SevCommonState *sev_common = SEV_COMMON(MACHINE(qdev_get_machine())->cgs);
>     if (sev_common->state == SEV_STATE_UNINIT) {
>@@ -2734,86 +2733,57 @@ static int cgs_set_guest_policy(ConfidentialGuestPolicyType policy_type,
>         return 0;
>     }
>
>-    if (policy_type != GUEST_POLICY_SEV) {
>-        error_setg(errp, "SEV: Invalid guest policy type provided for SEV: %d",
>-                   policy_type);
>+    if (!sev_snp_enabled()) {
>+        error_setg(errp, "SEV: ID block is only supported for SEV-SNP");
>         return -1;
>     }
>-    /*
>-     * SEV-SNP handles policy differently. The policy flags are defined in
>-     * kvm_start_conf.policy and an ID block and ID auth can be provided.
>-     */
>-    if (sev_snp_enabled()) {
>-        SevSnpGuestState *sev_snp_guest =
>-            SEV_SNP_GUEST(MACHINE(qdev_get_machine())->cgs);
>-        struct kvm_sev_snp_launch_finish *finish =
>-            &sev_snp_guest->kvm_finish_conf;
>
>-        /*
>-         * The policy consists of flags in 'policy' and optionally an ID block
>-         * and ID auth in policy_data1 and policy_data2 respectively. The ID
>-         * block and auth are optional so clear any previous ID block and auth
>-         * and set them if provided, but always set the policy flags.
>-         */
>-        g_free(sev_snp_guest->id_block);
>-        g_free((guchar *)finish->id_block_uaddr);
>-        g_free(sev_snp_guest->id_auth);
>-        g_free((guchar *)finish->id_auth_uaddr);
>-        sev_snp_guest->id_block = NULL;
>-        finish->id_block_uaddr = 0;
>-        sev_snp_guest->id_auth = NULL;
>-        finish->id_auth_uaddr = 0;
>-
>-        if (policy_data1_size > 0) {
>-            struct sev_snp_id_authentication *id_auth =
>-                (struct sev_snp_id_authentication *)policy_data2;
>-
>-            if (policy_data1_size != KVM_SEV_SNP_ID_BLOCK_SIZE) {
>-                error_setg(errp, "SEV: Invalid SEV-SNP ID block: incorrect size");
>-                return -1;
>-            }
>-            if (policy_data2_size != KVM_SEV_SNP_ID_AUTH_SIZE) {
>-                error_setg(errp,
>-                           "SEV: Invalid SEV-SNP ID auth block: incorrect size");
>-                return -1;
>-            }
>-            assert(policy_data1 != NULL);
>-            assert(policy_data2 != NULL);
>+    SevSnpGuestState *sev_snp_guest =
>+        SEV_SNP_GUEST(MACHINE(qdev_get_machine())->cgs);
>+    struct kvm_sev_snp_launch_finish *finish =
>+        &sev_snp_guest->kvm_finish_conf;
>
>-            finish->id_block_uaddr =
>-                (__u64)g_memdup2(policy_data1, KVM_SEV_SNP_ID_BLOCK_SIZE);
>-            finish->id_auth_uaddr =
>-                (__u64)g_memdup2(policy_data2, KVM_SEV_SNP_ID_AUTH_SIZE);
>+    g_free(sev_snp_guest->id_block);
>+    g_free((guchar *)finish->id_block_uaddr);
>+    g_free(sev_snp_guest->id_auth);
>+    g_free((guchar *)finish->id_auth_uaddr);
>+    sev_snp_guest->id_block = NULL;
>+    finish->id_block_uaddr = 0;
>+    sev_snp_guest->id_auth = NULL;
>+    finish->id_auth_uaddr = 0;
>
>-            /*
>-             * Check if an author key has been provided and use that to flag
>-             * whether the author key is enabled. The first of the author key
>-             * must be non-zero to indicate the key type, which will currently
>-             * always be 2.
>-             */
>-            sev_snp_guest->kvm_finish_conf.auth_key_en =
>-                id_auth->author_key[0] ? 1 : 0;
>-            finish->id_block_en = 1;
>-        }
>+    if (id_block_size > 0) {
>+        struct sev_snp_id_authentication *auth =
>+            (struct sev_snp_id_authentication *)id_auth;
>
>-        /* do not reset existing policy if policy was not set in IGVM  */
>-        if (policy != 0) {
>-            sev_snp_guest->kvm_start_conf.policy = policy;
>+        if (id_block_size != KVM_SEV_SNP_ID_BLOCK_SIZE) {
>+            error_setg(errp, "SEV: Invalid SEV-SNP ID block: incorrect size");
>+            return -1;
>         }
>-    } else {
>-        SevGuestState *sev_guest = SEV_GUEST(MACHINE(qdev_get_machine())->cgs);
>-        /* Only the policy flags are supported for SEV and SEV-ES */
>-        if ((policy_data1_size > 0) || (policy_data2_size > 0) || !sev_guest) {
>-            error_setg(errp, "SEV: An ID block/ID auth block has been provided "
>-                             "but SEV-SNP is not enabled");
>+        if (id_auth_size != KVM_SEV_SNP_ID_AUTH_SIZE) {
>+            error_setg(errp,
>+                       "SEV: Invalid SEV-SNP ID auth block: incorrect size");
>             return -1;
>         }
>+        assert(id_block != NULL);
>+        assert(id_auth != NULL);
>
>-        /* do not reset existing policy if policy was not set in IGVM  */
>-        if (policy != 0) {
>-            sev_guest->policy = policy;
>-        }
>+        finish->id_block_uaddr =
>+            (__u64)g_memdup2(id_block, KVM_SEV_SNP_ID_BLOCK_SIZE);
>+        finish->id_auth_uaddr =
>+            (__u64)g_memdup2(id_auth, KVM_SEV_SNP_ID_AUTH_SIZE);
>+
>+        /*
>+         * Check if an author key has been provided and use that to flag
>+         * whether the author key is enabled. The first of the author key
>+         * must be non-zero to indicate the key type, which will currently
>+         * always be 2.
>+         */
>+        sev_snp_guest->kvm_finish_conf.auth_key_en =
>+            auth->author_key[0] ? 1 : 0;
>+        finish->id_block_en = 1;
>     }
>+
>     return 0;
> }
>
>@@ -2878,7 +2848,7 @@ sev_common_instance_init(Object *obj)
>     cgs->check_support = cgs_check_support;
>     cgs->set_guest_state = cgs_set_guest_state;
>     cgs->get_mem_map_entry = cgs_get_mem_map_entry;
>-    cgs->set_guest_policy = cgs_set_guest_policy;
>+    cgs->set_id_block = cgs_set_id_block;
>     cgs->can_rebuild_guest_state = true;
>
>     QTAILQ_INIT(&sev_common->launch_vmsa);
>
>-- 
>2.55.0
>