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Date:   Mon, 27 Nov 2017 22:21:39 +0800
From:   Chunyu Hu <>
Subject: [PATCH] x86/idt: load idt early in start_secondary

For ap, idt is first loaded in cpu_init() with load_current_idt(),
that is to say, no exception can be handled before there. And then
the idt_table has been completed by the bp.

While there are some WARNs which needs the UD exception handling
in the early boot code might be triggered when something uexpected
happens during boot. In that case, cpu would fail to boot as the
exception can't be handled. A WARNing during boot is not usually
meaning the system could not boot.

One use case is when ftrace=function is setup in kernel cmdline, the
ftrace callback function will be called for every traced function.
And in my case, the first traced function is load_ucode_ap. And there
are WARN()s in function trace callback handling, it failed to reboot
as one of the WARN()s is triggered before load_current_idt() executed.

To make WARN()s can work earlier to ap, we load the idt_table early
in start_secondary, and keep the second time idt load in cpu_init,
as there is a load_ucode_ap() there.

Signed-off-by: Chunyu Hu <>
 arch/x86/kernel/smpboot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 3d01df7..05a97d5 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -237,7 +237,7 @@ static void notrace start_secondary(void *unused)
+	load_current_idt();

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