linux-kernel - Re: [RFC v2 PATCH 6/7] x86/entry: get rid of ALLOC_PT_GPREGS_ON_STACK and SAVE_AND_CLEAR

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Date:   Wed, 7 Feb 2018 13:58:20 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: [RFC v2 PATCH 6/7] x86/entry: get rid of ALLOC_PT_GPREGS_ON_STACK
 and SAVE_AND_CLEAR_REGS

On Wed, Feb 7, 2018 at 1:29 PM, Dominik Brodowski
<linux@...inikbrodowski.net> wrote:
>>
>> So this removes lines of asm code, but it adds a lot of instructions
>> to the end result thanks to the macro, I think.
>
> Indeed, that is the case (see below). However, if we want to switch to
> PUSH instructions and do this in a routine which is call'ed and which
> ret'urns, %rsp needs to be moved around even more often than the old
> ALLOC_PT_GPREGS_ON_STACK macro did (which you wanted to get rid of,
> IIUYC). Or do I miss something?

So I agree that your approach makes for a lot simpler stack setup.

I was just hoping that we could play some tricks.

For example, right now your PUSH_AND_CLEAR_REGS starts off with

        pushq   %rdi            /* pt_regs->di */
        pushq   %rsi            /* pt_regs->si */
        pushq   %rdx            /* pt_regs->dx */
        pushq   %rcx            /* pt_regs->cx */
        ....

and maybe we could still use this in paranoid_entry and error_entry if
we made it something like

/* if 'save_ret' is set, we pop the return point into %rsi */
.macro PUSH_AND_CLEAR_REGS save_ret=0
        .if \save_ret
        pushq %%rsi
        movq 8(%%rsp),%rsi
        movq %%rdi,8(%%rsp)
        .else
        pushq   %rdi            /* pt_regs->di */
        pushq   %rsi            /* pt_regs->si */
        .endif
        pushq   %rdx            /* pt_regs->dx */
        pushq   %rcx            /* pt_regs->cx */
        ....

which would allow error_entry and paranoid_entry to do something like this:

        PUSH_AND_CLEAR_REGS save_ret=1
        pushq %rsi
        ... do the other common code ..
        ret

(totally untested, I'm just doing a "stream-of-consciousness" thing in
the email.

See what I'm saying?

That said, maybe the pushq sequence is now so small that it doesn't
even matter, and duplicating it isn't a big problem.

Because your version sure is simpler.

         Linus