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

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Date:   Thu, 4 Jun 2020 15:25:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     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>,
        Paolo Bonzini <pbonzini@...hat.com>,
        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 Wed, Jun 03, 2020 at 08:18:44PM +0100, Andrew Cooper wrote:

> Well that didn't last long...
> 
> The new ISE (rev 39, published today) introduces BUS LOCK DEBUG
> EXCEPTION which is now a second inverted polarity sticky bit (bit 11) in
> %dr6.
> 
> This one is liable to get more traction than RTM debugging, so something
> probably does want fixing in the #DB handler.

Well that's crap :-(

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.

Who sodding throught this was a good idea ?! What happened to #AC that
SLD currently uses?

What hardware will this be in and can we get this fixed?