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

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Date:   Tue, 12 Apr 2022 21:52:44 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     LKML <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 ML <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 9:26 PM Borislav Petkov <bp@...en8.de> wrote:
>
> 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?

For a normal function, e.g. a function complied from a C function,
the stack will return to its original place when the function
returns.  The size of memory pushed and popped are the same in
a function.

>
> > 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.

The mapped size for the text is always 2M when the kernel is booted
since it is 2M-aligned.  So I don't think the size is a concern.

The only concern is the footprint when different interrupts and
exceptions happen heavily at the same time.  In this case, different
copies of PUSH_AND_CLEAR_REGS in the text will be touched.

For example, a heavy page fault and IPI or timer at the same time.
I'm not sure if it is a real case.

I'm Okay with asm_error_entry().  And also we can use an ASM function
containing PUSH_AND_CLEAR_REGS only.

Thanks
Lai