Improve the description of multi-processor setup

2 files changed, 18 insertions, 2 deletions

diff --git a/arch/x86/machine/cpu.h b/arch/x86/machine/cpu.h
index ab31e5d8..cf2c112b 100644
--- a/arch/x86/machine/cpu.h
+++ b/arch/x86/machine/cpu.h
@@ -615,6 +615,13 @@ void cpu_mp_probe(void);
 
 /*
  * Start application processors.
+ *
+ * The x86 architecture uses per-CPU page tables, which are created as a
+ * side effect of this function. In order to synchronize their page tables,
+ * processors must be able to communicate very soon after calling this
+ * function. They communicate through interrupts and threading facilities.
+ * On return, physical mappings must not be altered until inter-processor
+ * communication is available.
  */
 void cpu_mp_setup(void);
 
diff --git a/kern/kernel.c b/kern/kernel.c
index ea11bf2b..62d75b52 100644
--- a/kern/kernel.c
+++ b/kern/kernel.c
@@ -39,8 +39,17 @@ kernel_main(void)
     llsync_setup();
 
     /*
-     * Enabling application processors must be the last step before starting
-     * the scheduler.
+     * Enabling application processors is done late in the boot process for
+     * two reasons :
+     *  - It's much simpler to bootstrap with interrupts disabled on all
+     *    processors, enabling them only when necessary on the BSP.
+     *  - Depending on the architecture, the pmap module could create per
+     *    processor page tables. Once done, keeping the kernel page tables
+     *    synchronized requires interrupts (and potentially scheduling)
+     *    enabled on all processors.
+     *
+     * Anything done after this call and before running the scheduler must
+     * not alter physical mappings.
      */
     cpu_mp_setup();