cxl/mbox: Add variable output size validation for internal commands

cxl_internal_send_cmd() skips output size validation for variable output
commands which is not ideal. Most of the time internal usages want to
fail if the output size does not match what was requested. For other
commands where the caller cannot predict the size there is usually a
a header that conveys how much vaild data is in the payload. For those
cases add @min_out as a parameter to specify what the minimum response
payload needs to be for the caller to parse the rest of the payload.

In this patch only Get Supported Logs has that behavior, but going
forward records retrieval commands like Get Poison List and Get Event
Records can use @min_out to retrieve a variable amount of records.

Critically, this validation scheme skips the needs to interrogate the
cxl_mem_commands array which in turn frees up the implementation to
support internal command enabling without also enabling external / user
commands.
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/167030055918.4044561.10339573829837910505.stgit@dwillia2-xfh.jf.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/cxl/core/mbox.c

@@ -166,9 +166,7 @@ static const char *cxl_mem_opcode_to_name(u16 opcode)
 int cxl_internal_send_cmd(struct cxl_dev_state *cxlds,
 			  struct cxl_mbox_cmd *mbox_cmd)
 {
-	const struct cxl_mem_command *cmd =
-		cxl_mem_find_command(mbox_cmd->opcode);
-	size_t out_size;
+	size_t out_size, min_out;
 	int rc;
 
 	if (mbox_cmd->size_in > cxlds->payload_size ||
@@ -176,6 +174,7 @@ int cxl_internal_send_cmd(struct cxl_dev_state *cxlds,
 		return -E2BIG;
 
 	out_size = mbox_cmd->size_out;
+	min_out = mbox_cmd->min_out;
 	rc = cxlds->mbox_send(cxlds, mbox_cmd);
 	if (rc)
 		return rc;
@@ -183,14 +182,18 @@ int cxl_internal_send_cmd(struct cxl_dev_state *cxlds,
 	if (mbox_cmd->return_code != CXL_MBOX_CMD_RC_SUCCESS)
 		return cxl_mbox_cmd_rc2errno(mbox_cmd);
 
+	if (!out_size)
+		return 0;
+
 	/*
-	 * Variable sized commands can't be validated and so it's up to the
-	 * caller to do that if they wish.
+	 * Variable sized output needs to at least satisfy the caller's
+	 * minimum if not the fully requested size.
 	 */
-	if (cmd->info.size_out != CXL_VARIABLE_PAYLOAD) {
-		if (mbox_cmd->size_out != out_size)
-			return -EIO;
-	}
+	if (min_out == 0)
+		min_out = out_size;
+
+	if (mbox_cmd->size_out < min_out)
+		return -EIO;
 	return 0;
 }
 EXPORT_SYMBOL_NS_GPL(cxl_internal_send_cmd, CXL);
@@ -635,6 +638,8 @@ static struct cxl_mbox_get_supported_logs *cxl_get_gsl(struct cxl_dev_state *cxl
 		.opcode = CXL_MBOX_OP_GET_SUPPORTED_LOGS,
 		.size_out = cxlds->payload_size,
 		.payload_out = ret,
+		/* At least the record number field must be valid */
+		.min_out = 2,
 	};
 	rc = cxl_internal_send_cmd(cxlds, &mbox_cmd);
 	if (rc < 0) {

drivers/cxl/cxlmem.h

@@ -101,6 +101,7 @@ static inline struct cxl_ep *cxl_ep_load(struct cxl_port *port,
  *            outputs commands this is always expected to be deterministic. For
  *            variable sized output commands, it tells the exact number of bytes
  *            written.
+ * @min_out: (input) internal command output payload size validation
  * @return_code: (output) Error code returned from hardware.
  *
  * This is the primary mechanism used to send commands to the hardware.
@@ -115,6 +116,7 @@ struct cxl_mbox_cmd {
 	void *payload_out;
 	size_t size_in;
 	size_t size_out;
+	size_t min_out;
 	u16 return_code;
 };