Re: [tip:x86/urgent] x86/boot/64: Use 'push' instead of 'call' in start_cpu()

From: hpa
Date: Wed Dec 14 2016 - 14:27:08 EST


On December 14, 2016 12:36:58 AM PST, tip-bot for Josh Poimboeuf <tipbot@xxxxxxxxx> wrote:
>Commit-ID: ec2d86a9b646d93f1948569f368e2c6f5449e6c7
>Gitweb:
>http://git.kernel.org/tip/ec2d86a9b646d93f1948569f368e2c6f5449e6c7
>Author: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>AuthorDate: Tue, 13 Dec 2016 21:25:35 -0600
>Committer: Ingo Molnar <mingo@xxxxxxxxxx>
>CommitDate: Wed, 14 Dec 2016 08:48:05 +0100
>
>x86/boot/64: Use 'push' instead of 'call' in start_cpu()
>
>start_cpu() pushes a text address on the stack so that stack traces
>from
>idle tasks will show start_cpu() at the end. But it uses a call
>instruction to do that, which is rather obtuse. Use a straightforward
>push instead.
>
>Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
>Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>Cc: Brian Gerst <brgerst@xxxxxxxxx>
>Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
>Cc: H. Peter Anvin <hpa@xxxxxxxxx>
>Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>Link:
>http://lkml.kernel.org/r/4d8a1952759721d42d1e62ba9e4a7e3ac5df8574.1481685203.git.jpoimboe@xxxxxxxxxx
>Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
>---
> arch/x86/kernel/head_64.S | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
>index 90de288..1facaf4 100644
>--- a/arch/x86/kernel/head_64.S
>+++ b/arch/x86/kernel/head_64.S
>@@ -298,7 +298,7 @@ ENTRY(start_cpu)
> * REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
> * address given in m16:64.
> */
>- call 1f # put return address on stack for unwinder
>+ pushq $1f # put return address on stack for unwinder
> 1: xorq %rbp, %rbp # clear frame pointer
> movq initial_code(%rip), %rax
> pushq $__KERNEL_CS # set correct cs

This adds another relocation to the kernel. I hope this is safe at this point in the code?
--
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