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.

Though this SEV feature is called "Debug virtualization" in the APM, KVM
calls this "debug swap" so use the same name for consistency.

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

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
---
 target/i386/sev.h |  1 +
 target/i386/sev.c | 20 ++++++++++++++++++++
 qapi/qom.json     |  6 +++++-
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/target/i386/sev.h b/target/i386/sev.h
index 102546b112d6..8e09b2ce1976 100644
--- a/target/i386/sev.h
+++ b/target/i386/sev.h
@@ -45,6 +45,7 @@ bool sev_snp_enabled(void);
 #define SEV_SNP_POLICY_DBG      0x80000
 
 #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 88dd0750d481..e9d84ea25571 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -319,6 +319,11 @@ 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 set)
 {
     if (set) {
@@ -2744,6 +2749,16 @@ 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)
 {
@@ -2761,6 +2776,11 @@ 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..df962d4a5215 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:
-- 
2.51.0