Add support for enabling debug-swap VMSA SEV feature in SEV-ES and
SEV-SNP guests through a new "debug-swap" boolean property on SEV guest
objects. Though the boolean property is available for plain SEV guests,
check_sev_features() will reject setting this for plain SEV guests.

Add helpers for setting and querying the VMSA SEV features so that they
can be re-used for subsequent VMSA SEV features, and convert the
existing SVM_SEV_FEAT_SNP_ACTIVE definition to use the BIT() macro for
consistency with the new feature flag.

Sample command-line:
  -machine q35,confidential-guest-support=sev0 \
  -object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1,debug-swap=on

Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
---
 target/i386/sev.h |  3 ++-
 target/i386/sev.c | 29 +++++++++++++++++++++++++++++
 qapi/qom.json     |  6 +++++-
 3 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/target/i386/sev.h b/target/i386/sev.h
index 9db1a802f6bb..8e09b2ce1976 100644
--- a/target/i386/sev.h
+++ b/target/i386/sev.h
@@ -44,7 +44,8 @@ bool sev_snp_enabled(void);
 #define SEV_SNP_POLICY_SMT      0x10000
 #define SEV_SNP_POLICY_DBG      0x80000
 
-#define SVM_SEV_FEAT_SNP_ACTIVE 1
+#define SVM_SEV_FEAT_SNP_ACTIVE     BIT(0)
+#define SVM_SEV_FEAT_DEBUG_SWAP     BIT(5)
 
 typedef struct SevKernelLoaderContext {
     char *setup_data;
diff --git a/target/i386/sev.c b/target/i386/sev.c
index fa23b5c38e9b..b3e4d0f2c1d5 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -319,6 +319,20 @@ sev_set_guest_state(SevCommonState *sev_common, SevState new_state)
     sev_common->state = new_state;
 }
 
+static bool is_sev_feature_set(SevCommonState *sev_common, uint64_t feature)
+{
+    return !!(sev_common->sev_features & feature);
+}
+
+static void sev_set_feature(SevCommonState *sev_common, uint64_t feature, bool value)
+{
+    if (value) {
+        sev_common->sev_features |= feature;
+    } else {
+        sev_common->sev_features &= ~feature;
+    }
+}
+
 static void
 sev_ram_block_added(RAMBlockNotifier *n, void *host, size_t size,
                     size_t max_size)
@@ -2732,6 +2746,16 @@ static int cgs_set_guest_policy(ConfidentialGuestPolicyType policy_type,
     return 0;
 }
 
+static bool sev_common_get_debug_swap(Object *obj, Error **errp)
+{
+    return is_sev_feature_set(SEV_COMMON(obj), SVM_SEV_FEAT_DEBUG_SWAP);
+}
+
+static void sev_common_set_debug_swap(Object *obj, bool value, Error **errp)
+{
+    sev_set_feature(SEV_COMMON(obj), SVM_SEV_FEAT_DEBUG_SWAP, value);
+}
+
 static void
 sev_common_class_init(ObjectClass *oc, const void *data)
 {
@@ -2749,6 +2773,11 @@ sev_common_class_init(ObjectClass *oc, const void *data)
                                    sev_common_set_kernel_hashes);
     object_class_property_set_description(oc, "kernel-hashes",
             "add kernel hashes to guest firmware for measured Linux boot");
+    object_class_property_add_bool(oc, "debug-swap",
+                                   sev_common_get_debug_swap,
+                                   sev_common_set_debug_swap);
+    object_class_property_set_description(oc, "debug-swap",
+            "enable virtualization of debug registers");
 }
 
 static void
diff --git a/qapi/qom.json b/qapi/qom.json
index 830cb2ffe781..71cd8ad588b5 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -1010,13 +1010,17 @@
 #     designated guest firmware page for measured boot with -kernel
 #     (default: false) (since 6.2)
 #
+# @debug-swap: enable virtualization of debug registers (default: false)
+#              (since 10.2)
+#
 # Since: 9.1
 ##
 { 'struct': 'SevCommonProperties',
   'data': { '*sev-device': 'str',
             '*cbitpos': 'uint32',
             'reduced-phys-bits': 'uint32',
-            '*kernel-hashes': 'bool' } }
+            '*kernel-hashes': 'bool',
+            '*debug-swap': 'bool' } }
 
 ##
 # @SevGuestProperties:
-- 
2.50.1