linux-kernel - Re: [patch V2 07/17] x86/entry/64: Remove redundant interrupt disable

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Message-ID: <20191024212105.GY4114@hirez.programming.kicks-ass.net>
Date:   Thu, 24 Oct 2019 23:21:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Will Deacon <will@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [patch V2 07/17] x86/entry/64: Remove redundant interrupt disable

On Thu, Oct 24, 2019 at 10:52:59PM +0200, Thomas Gleixner wrote:
> is just undefined behaviour and I personally consider it to be a plain bug.

I concur.

> Just for the record: This results in running a resulting or even completely
> unrelated signal handler with interrupts disabled as well.
> 
> Whatever we decide it is, leaving it completely inconsistent is not a
> solution at all. The options are:
> 
>   1)  Always do conditional tracing depending on the user_regs->eflags.IF
>       state.
> 
>   2)  #1 + warn once when syscalls and exceptions (except NMI/MCE) happen
>       and user_regs->eflags.IF is cleared.
> 
>   3a) #2 + enforce signal handling to run with interrupts enabled.
> 
>   3b) #2 + set regs->eflags.IF. So the state is always correct from the
>       kernel POV. Of course that changes existing behaviour, but its
>       changing undefined and inconsistent behaviour.
>   
>   4) Let iopl(level) return -EPERM if level == 3.
> 
>      Yeah, I know it's not possible due to regressions (DPKD uses iopl(3)),
>      but TBH that'd be the sanest option of all.
> 
>      Of course the infinite wisdom of hardware designers tied IN, INS, OUT,
>      OUTS and CLI/STI together on IOPL so we cannot even distangle them in
>      any way.
> 
>      The only way out would be to actually use a full 8K sized I/O bitmap,
>      but that's a massive pain as it has to be copied on every context
>      switch. 
> 
> Really pretty options to chose from ...

If 4 is out (and I'm afraid it might be), then I'm on record for liking
3b.