The metadata field readers get_tdx_sys_info_<class>() in
tdx_global_metadata.c were generated by an out-of-tree script. That has
not worked out: the JSON file they were generated from is neither stable
nor authoritative enough [1].  The goal now is to maintain the readers by
hand and to establish one standard way of adding a metadata field.

Take get_tdx_sys_info_version() as an example:

      if (!ret && !(ret = read_sys_metadata_field(0x0800000100000003, &val)))
              sysinfo_version->minor_version = val;
      if (!ret && !(ret = read_sys_metadata_field(0x0800000100000004, &val)))
              sysinfo_version->major_version = val;
      if (!ret && !(ret = read_sys_metadata_field(0x0800000100000005, &val)))
              sysinfo_version->update_version = val;

Two patterns stand out: the read-check-store sequence repeats once per
field, and the error of each read is chained into the reads that follow.
Neither is common in hand-written code.

Prepare to eliminate both with a loop that reads each field, stores the
value into its structure member, and returns on the first error.

Add 'struct field_mapping' to describe one field as its ID plus the offset
and size of the member that receives its value. Add FIELD_MAP() to
build such an entry from a field ID, a structure type and a member name.
Add __read_sys_metadata_table() helper to read every field in a table.
Annotate that helper __maybe_unused as there is no caller right now.

Following changes will convert the existing readers to use the new helper.

AI was used under supervision to review code and workshop logs. It
suggested adding read_sys_metadata_table() macro, which avoids repeating
the table name when passing both the table and its size.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://lore.kernel.org/kvm/1e7bcbad-eb26-44b7-97ca-88ab53467212@intel.com/ # [1]
---
v4:
 - Collect tags [Nikolay, Rick]
 - Rename TDX_SYSINFO_MAP() to FIELD_MAP() to keep table entries short
 - Builds the full structure name from its tdx_sys_info_ suffix [Yilun]

v3:
 - Pass the field ID to TDX_SYSINFO_MAP() instead of pasting the
   TDX_MD_FIELD_ID_ prefix onto a suffix inside the macro [Rick]
 - Say that this patch only prepares to eliminate the two patterns. [Rick]
 - Define @offset in struct field_mapping as size_t [Binbin]
---
 arch/x86/virt/vmx/tdx/tdx.c | 42 +++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 96ced0494b68..65a6aae6fab0 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -395,6 +395,48 @@ static int read_sys_metadata_field(u64 field_id, u64 *data)
 	return 0;
 }
 
+/*
+ * Map a TDX global metadata field to a structure member.
+ * @field_id: The TDX global metadata field ID.
+ * @size: The size of the structure member.
+ * @offset: The member's offset within its containing structure.
+ */
+struct field_mapping {
+	u64 field_id;
+	size_t size;
+	size_t offset;
+};
+
+/* Read each metadata field listed in @mappings[] into @data. */
+static int __maybe_unused __read_sys_metadata_table(const struct field_mapping *mappings,
+						    int num_mappings, void *data)
+{
+	int i, ret;
+	u64 val;
+
+	for (i = 0; i < num_mappings; i++) {
+		ret = read_sys_metadata_field(mappings[i].field_id, &val);
+		if (ret)
+			return ret;
+		memcpy((char *)data + mappings[i].offset, &val, mappings[i].size);
+	}
+
+	return 0;
+}
+
+#define read_sys_metadata_table(_mappings, _data)	\
+	__read_sys_metadata_table(_mappings, ARRAY_SIZE(_mappings), _data)
+
+#define __FIELD_MAP(_field, _type, _member)			\
+{								\
+	.field_id	= _field,				\
+	.offset		= offsetof(_type, _member),		\
+	.size		= sizeof_field(_type, _member),		\
+}
+
+#define FIELD_MAP(_field, _type, _member)	\
+	__FIELD_MAP(_field, struct tdx_sys_info_##_type, _member)
+
 #include "tdx_global_metadata.c"
 
 static __init int check_features(struct tdx_sys_info *sysinfo)
-- 
2.52.0