linux-kernel - Re: [patch V9 00/39] x86/entry: Rework leftovers (was part V)

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Date:   Thu, 4 Jun 2020 08:42:52 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Wei Liu <wei.liu@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Zhao Yakui <yakui.zhao@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch V9 00/39] x86/entry: Rework leftovers (was part V)

On Thu, Jun 4, 2020 at 6:35 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Jun 04, 2020 at 03:29:26PM +0200, Paolo Bonzini wrote:
> > On 04/06/20 15:25, Peter Zijlstra wrote:
> > > It being enabled through IA32_DEBUGCTL instead of through DR7 means that
> > > the current code doesn't disable it and this then means we can have
> > > nested #DB again.
> >
> > /me bangs head on door
> >
> > > Who sodding throught this was a good idea ?! What happened to #AC that
> > > SLD currently uses?
> >
> > It was per-core and (presumably) considered unfixable?
>
> Yeah, but I don't see how changing the exception vector helps with that.
> #DB is an IST, and it must be, because of that lovely MOV SS thing. #AC
> has none of that, _please_ use #AC.

x86 is not an architecture.  x86 is a gauntlet through which operating
system developers must run.

I think we can tolerate this particular mess -- can't we just say that
a BUS LOCK DEBUG EXCEPTION is fatal if it came from kernel mode?  So
what if it nests inside #DB -- we can survive an oops from a corrupt
context like that.