linux-kernel - [PATCH] x86/entry/64: Simplify ENCODE_FRAME

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Date:   Tue, 20 Feb 2018 20:42:14 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dominik Brodowski <linux@...inikbrodowski.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>
Subject: [PATCH] x86/entry/64: Simplify ENCODE_FRAME_POINTER

On Tue, Feb 20, 2018 at 02:25:03PM -0800, Linus Torvalds wrote:
> On Tue, Feb 20, 2018 at 1:01 PM, Dominik Brodowski
> <linux@...inikbrodowski.net> wrote:
> > +ENTRY(interrupt_entry)
> > +       UNWIND_HINT_FUNC
> > +
> > +       PUSH_AND_CLEAR_REGS save_ret=1
> > +       ENCODE_FRAME_POINTER 8
> > +
> > +       ret
> > +END(interrupt_entry)
> 
> There's nothing wrong with this patch, but it does expose what a nasty
> hack our "ENCODE_FRAME_POINTER" thing is.
> 
> It generates (when there is an offset, like this):
> 
>         leaq \ptregs_offset(%rsp), %rbp
>         orq     $0x1, %rbp
> 
> and I would _really_ hope that the stack pointer is always aligned on
> interrupt entry, so I don't see why it's not just
> 
>         leaq 1+\ptregs_offset(%rsp), %rbp
> 
> instead.
> 
> I dunno. Let's ask Josh what the reason for the separate "or" was.
> 
> But this is an independent issue of this patch, really.

Indeed, how about this?

----

From: Josh Poimboeuf <jpoimboe@...hat.com>
Subject: [PATCH] x86/entry/64: Simplify ENCODE_FRAME_POINTER

On 64-bit, the stack pointer is always aligned on interrupt, so instead
of setting the LSB of the pt_regs address, we can just add 1 to it.

Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 arch/x86/entry/calling.h | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index dce7092ab24a..400cea8bbf4b 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -172,12 +172,7 @@ For 32-bit we have the following conventions - kernel is built with
  */
 .macro ENCODE_FRAME_POINTER ptregs_offset=0
 #ifdef CONFIG_FRAME_POINTER
-	.if \ptregs_offset
-		leaq \ptregs_offset(%rsp), %rbp
-	.else
-		mov %rsp, %rbp
-	.endif
-	orq	$0x1, %rbp
+	leaq 1+\ptregs_offset(%rsp), %rbp
 #endif
 .endm
 
-- 
2.14.3