target/i386: SEV: Refactor check_sev_features()

Refactor check_sev_features() to consolidate SEV-SNP checks to a single
if block. This is also helpful when adding checks for future SEV
features.  While at it, move the comment about the checks being done
outside of the function body and expand it to describe what this
function does. Update error_setg() invocations to use a consistent
format.

No functional change intended.

Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/cae04d88adfbcdc2997e518475f3b89091adf8a9.1779281646.git.naveen@kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/sev.c

diff --git a/target/i386/sev.c b/target/i386/sev.c
index ae9c85a..8acbb01 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -507,40 +507,22 @@
     }
 }
 
+/*
+ * Ensure SEV_FEATURES is configured for correct SEV hardware and that
+ * the requested features are supported. In addition, ensure feature
+ * dependencies are satisfied (allow tsc-frequency only if secure-tsc
+ * is also enabled, as an example).
+ */
 static int check_sev_features(SevCommonState *sev_common, uint64_t sev_features,
                               Error **errp)
 {
-    /*
-     * Ensure SEV_FEATURES is configured for correct SEV hardware and that
-     * the requested features are supported. If SEV-SNP is enabled then
-     * that feature must be enabled, otherwise it must be cleared.
-     */
-    if (sev_snp_enabled() && !(sev_features & SVM_SEV_FEAT_SNP_ACTIVE)) {
-        error_setg(
-            errp,
-            "%s: SEV_SNP is enabled but is not enabled in VMSA sev_features",
-            __func__);
-        return -1;
-    } else if (!sev_snp_enabled() &&
-               (sev_features & SVM_SEV_FEAT_SNP_ACTIVE)) {
-        error_setg(
-            errp,
-            "%s: SEV_SNP is not enabled but is enabled in VMSA sev_features",
-            __func__);
-        return -1;
-    }
-    if (sev_features && sev_es_enabled() && !sev_snp_enabled()) {
-        error_setg(errp,
-                   "%s: SEV features are not supported with SEV-ES at this time",
-                   __func__);
-        return -1;
-    }
     if (sev_features && !sev_es_enabled()) {
         error_setg(errp,
                    "%s: SEV features require either SEV-ES or SEV-SNP to be enabled",
                    __func__);
         return -1;
     }
+
     if (sev_features & ~sev_common->supported_sev_features) {
         error_setg(errp,
                    "%s: VMSA contains unsupported sev_features: %lX, "
@@ -548,13 +530,36 @@
                    __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;
+
+    if (sev_snp_enabled()) {
+        if (!(sev_features & SVM_SEV_FEAT_SNP_ACTIVE)) {
+            error_setg(errp,
+                       "%s: SEV_SNP is enabled but is not enabled in VMSA sev_features",
+                       __func__);
+            return -1;
+        }
+        if (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;
+        }
+    } else {
+        if (sev_features && sev_es_enabled()) {
+            error_setg(errp,
+                       "%s: SEV features are not supported with SEV-ES at this time",
+                       __func__);
+            return -1;
+        }
+        if (sev_features & SVM_SEV_FEAT_SNP_ACTIVE) {
+            error_setg(errp,
+                       "%s: SEV_SNP is not enabled but is enabled in VMSA sev_features",
+                       __func__);
+            return -1;
+        }
     }
+
     return 0;
 }