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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 4 Jun 2020 08:42:52 -0700
Message-ID: <CALCETrV89Sat5Dm5YTF_pcxMMrxygEwa78trpONm8MVMGZtWPg@mail.gmail.com>
Subject: Re: [patch V9 00/39] x86/entry: Rework leftovers (was part V)
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Wei Liu <wei.liu@kernel.org>,
        Michael Kelley <mikelley@microsoft.com>,
        Jason Chen CJ <jason.cj.chen@intel.com>,
        Zhao Yakui <yakui.zhao@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
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On Thu, Jun 4, 2020 at 6:35 AM Peter Zijlstra <peterz@infradead.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.