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Message-ID: <Yk3jVrXoVpxuR0Mp@zn.tnic>
Date: Wed, 6 Apr 2022 21:00:38 +0200
From: Borislav Petkov <bp@...en8.de>
To: Lai Jiangshan <jiangshanlai@...il.com>
Cc: linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Andy Lutomirski <luto@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
Lai Jiangshan <jiangshan.ljs@...group.com>,
Ingo Molnar <mingo@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>,
Fenghua Yu <fenghua.yu@...el.com>,
Thomas Tai <thomas.tai@...cle.com>,
"Chang S. Bae" <chang.seok.bae@...el.com>,
Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH V4 1/7] x86/traps: Move pt_regs only in fixup_bad_iret()
On Fri, Mar 18, 2022 at 10:30:10PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@...group.com>
>
> fixup_bad_iret() and sync_regs() have similar arguments and do similar
> work that copies full or partial pt_regs to a place and switches stack
> after return. They are quite the same, but fixup_bad_iret() not only
> copies the pt_regs but also the return address of error_entry() while
What return address of error_entry()? You lost me here.
fixup_bad_iret() moves the stack frame while sync_regs() switches to the
thread stack. I have no clue what you mean.
> sync_regs() copies the pt_regs only and the return address of
> error_entry() was preserved and handled in ASM code.
Nope, no idea.
> This patch makes fixup_bad_iret() work like sync_regs() and the
Avoid having "This patch" or "This commit" in the commit message. It is
tautologically useless.
Also, do
$ git grep 'This patch' Documentation/process
for more details.
> handling of the return address of error_entry() is moved in ASM code.
>
> It removes the need to use the struct bad_iret_stack, simplifies
> fixup_bad_iret() and makes the ASM error_entry() call fixup_bad_iret()
> as the same as calling sync_regs() which adds readability because
> the calling patterns are exactly the same.
So fixup_bad_iret() gets the stack ptr passed in by doing:
mov %rsp, %rdi
call fixup_bad_iret
mov %rax, %rsp
and error_regs()
movq %rsp, %rdi /* arg0 = pt_regs pointer */
call sync_regs
movq %rax, %rsp /* switch stack */
the same way.
Confused.
> It is prepared for later patch to do the stack switch after the
> error_entry() which simplifies the code further.
Looking at your next patch, is all this dance done just so that you can
do
leaq 8(%rsp), %rdi
in order to pass in pt_regs to both functions?
And get rid of the saving/restoring %r12?
Is that what the whole noise is about?
--
Regards/Gruss,
Boris.
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