linux-kernel - Re: [PATCH V5 3/7] x86/entry: Move PUSH_AND_CLEAR_REGS out of error

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Message-ID: <YlV+B8faRD2lDoO3@zn.tnic>
Date:   Tue, 12 Apr 2022 15:26:31 +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>
Subject: Re: [PATCH V5 3/7] x86/entry: Move PUSH_AND_CLEAR_REGS out of
 error_entry()

On Tue, Apr 12, 2022 at 08:15:37PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@...group.com>
> 
> error_entry() doesn't handle the stack balanced.

What does that mean?

> It includes
> PUSH_AND_CLEAR_REGS which is commonly needed for all IDT entries and
> can't pop the regs before it returns.
> 
> Move PUSH_AND_CLEAR_REGS out of error_entry() and make error_entry()
> works on the stack normally.
> 
> After this, XENPV doesn't need error_entry() since PUSH_AND_CLEAR_REGS
> is moved out and error_entry() can be converted to C code in future
> since it doesn't fiddle the stack.

This is not a justification for this size increase:

   text    data     bss     dec     hex filename
16060616        128131358       36384888        180576862       ac3625e vmlinux.before
16065626        128131358       36380792        180577776       ac365f0 vmlinux.after

~5K text increase already with my tailored config.

You can have a asm_error_entry(), written in asm, which does the regs
pushing and which calls error_entry() - the latter being the C version.
And no need for the size increase.

For example.

-- 
Regards/Gruss,
    Boris.

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