Re: [PATCH V4 1/7] x86/traps: Move pt_regs only in fixup_bad_iret()

From: Borislav Petkov <bp@alien8.de>
To: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	x86@kernel.org, Lai Jiangshan <jiangshan.ljs@antgroup.com>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Thomas Tai <thomas.tai@oracle.com>,
	"Chang S. Bae" <chang.seok.bae@intel.com>,
	Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [PATCH V4 1/7] x86/traps: Move pt_regs only in fixup_bad_iret()
Date: Wed, 6 Apr 2022 21:00:38 +0200	[thread overview]
Message-ID: <Yk3jVrXoVpxuR0Mp@zn.tnic> (raw)
In-Reply-To: <20220318143016.124387-2-jiangshanlai@gmail.com>

On Fri, Mar 18, 2022 at 10:30:10PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@antgroup.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.

https://people.kernel.org/tglx/notes-about-netiquette