linux-kernel - Re: [PATCH V3.1] entry: Pass irqentry_state

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Message-ID: <878s9wshsa.fsf@nanos.tec.linutronix.de>
Date:   Thu, 17 Dec 2020 14:07:01 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>, Weiny Ira <ira.weiny@...el.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "open list\:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux-MM <linux-mm@...ck.org>,
        "open list\:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH V3.1] entry: Pass irqentry_state_t by reference

On Fri, Dec 11 2020 at 14:14, Andy Lutomirski wrote:
> On Mon, Nov 23, 2020 at 10:10 PM <ira.weiny@...el.com> wrote:
> After contemplating this for a bit, I think this isn't really the
> right approach.  It *works*, but we've mostly just created a bit of an
> unfortunate situation.  Our stack, on a (possibly nested) entry looks
> like:
>
> previous frame (or empty if we came from usermode)
> ---
> SS
> RSP
> FLAGS
> CS
> RIP
> rest of pt_regs
>
> C frame
>
> irqentry_state_t (maybe -- the compiler is within its rights to play
> almost arbitrary games here)
>
> more C stuff
>
> So what we've accomplished is having two distinct arch register
> regions, one called pt_regs and the other stuck in irqentry_state_t.
> This is annoying because it means that, if we want to access this
> thing without passing a pointer around or access it at all from outer
> frames, we need to do something terrible with the unwinder, and we
> don't want to go there.
>
> So I propose a somewhat different solution: lay out the stack like this.
>
> SS
> RSP
> FLAGS
> CS
> RIP
> rest of pt_regs
> PKS
> ^^^^^^^^ extended_pt_regs points here
>
> C frame
> more C stuff
> ...
>
> IOW we have:
>
> struct extended_pt_regs {
>   bool rcu_whatever;
>   other generic fields here;
>   struct arch_extended_pt_regs arch_regs;
>   struct pt_regs regs;
> };
>
> and arch_extended_pt_regs has unsigned long pks;
>
> and instead of passing a pointer to irqentry_state_t to the generic
> entry/exit code, we just pass a pt_regs pointer.

While I agree vs. PKS which is architecture specific state and needed in
other places e.g. #PF, I'm not convinced that sticking the existing
state into the same area buys us anything more than an indirect access.

Peter?

Thanks,

        tglx