debug: allow debug_early to work really early again

an effect of commit 9bbbe9dc is that no output is possible
before prio 'console' initializers have been run.
This change allows to have at least one API available
really early (from boot code and premain).
Document the requirements for the early console class
regarding the write() method.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

core/debug.cc

@@ -15,6 +15,8 @@
 #include <osv/debug.hh>
 #include <osv/debug.h>
 
+#include "early-console.hh"
+
 using namespace std;
 
 logger* logger::_instance = nullptr;
@@ -248,9 +250,19 @@ extern "C" {
         console::write_ll(msg, strlen(msg));
     }
 
+    /* Early Console Output
+     *
+     * For early output functionality (from boot code, premain) we need
+     * to access the write() method of the arch-specific early console
+     * directly, because cannot wait for the console framework to be
+     * initialized, not even its "early" functionality.
+     * Therefore the write() method of the early console needs
+     * to work without requiring any class construction.
+     */
+
     void debug_early(const char *msg)
     {
-        console::write_ll(msg, strlen(msg));
+        console::arch_early_console.write(msg, strlen(msg));
     }
 
     void debug_early_u64(const char *msg, unsigned long long val)
@@ -267,8 +279,8 @@ extern "C" {
             val >>= 4;
         }
 
-        console::write_ll(msg, strlen(msg));
-        console::write_ll(nr, 16);
-        console::write_ll("\n", 1);
+        console::arch_early_console.write(msg, strlen(msg));
+        console::arch_early_console.write(nr, 16);
+        console::arch_early_console.write("\n", 1);
     }
 }

loader.cc

@@ -98,6 +98,8 @@ extern "C" {
 
 void premain()
 {
+    debug_early_entry("early-console test: premain");
+
     arch_init_premain();
 
     auto inittab = elf::get_init(elf_header);