[RFC PATCH v2 01/10] x86/virt/tdx: Add a helper to read a table of metadata fields

[Chao Gao <chao.gao@intel.com>]

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.

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
TDX_SYSINFO_MAP() to build such an entry from a field ID name, a
structure type and a member name.  Add __read_sys_metadata_table() to
read every field in a table. Annotate the 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.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Link: https://lore.kernel.org/kvm/1e7bcbad-eb26-44b7-97ca-88ab53467212@intel.com/ # [1]
---
 arch/x86/virt/vmx/tdx/tdx.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 063574ed8625..5cd9c6eb98c2 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -395,6 +395,42 @@ 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;
+	int 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 TDX_SYSINFO_MAP(_field, _type, _member)			\
+{								\
+	.field_id	= TDX_MD_FIELD_ID_##_field,		\
+	.offset		= offsetof(_type, _member),		\
+	.size		= sizeof_field(_type, _member),		\
+}
+
 #include "tdx_global_metadata.c"
 
 static __init int check_features(struct tdx_sys_info *sysinfo)
-- 
2.52.0