[tip: x86/entry] x86/idt: Add comments about early #PF handling

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the x86/entry branch of tip:

Commit-ID:     66d2e706c0cecd09b1f3de4844574d30e5469c28
Gitweb:        https://git.kernel.org/tip/66d2e706c0cecd09b1f3de4844574d30e5469c28
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Thu, 28 May 2020 16:53:17 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Sat, 30 May 2020 11:50:11 +02:00

x86/idt: Add comments about early #PF handling

The difference between 32 and 64 bit vs. early #PF handling is not
documented. Replace the FIXME at idt_setup_early_pf() with proper comments.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200528145522.807135882@xxxxxxxxxxxxx

---
 arch/x86/kernel/idt.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index 4b99f7b..5ef82fc 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -61,7 +61,11 @@ static bool idt_setup_done __initdata;
 static const __initconst struct idt_data early_idts[] = {
 	INTG(X86_TRAP_DB,		asm_exc_debug),
 	SYSG(X86_TRAP_BP,		asm_exc_int3),
+
 #ifdef CONFIG_X86_32
+	/*
+	 * Not possible on 64-bit. See idt_setup_early_pf() for details.
+	 */
 	INTG(X86_TRAP_PF,		asm_exc_page_fault),
 #endif
 };
@@ -256,8 +260,10 @@ void __init idt_setup_traps(void)
  * cpu_init() is invoked and sets up TSS. The IST variant is installed
  * after that.
  *
- * FIXME: Why is 32bit and 64bit installing the PF handler at different
- * places in the early setup code?
+ * Note, that X86_64 cannot install the real #PF handler in
+ * idt_setup_early_traps() because the memory intialization needs the #PF
+ * handler from the early_idt_handler_array to initialize the early page
+ * tables.
  */
 void __init idt_setup_early_pf(void)
 {