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Date:   Thu, 4 Jun 2020 15:25:37 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andrew Cooper <andrew.cooper3@citrix.com>
Cc:     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>,
        Paolo Bonzini <pbonzini@redhat.com>,
        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>
Subject: Re: [patch V9 00/39] x86/entry: Rework leftovers (was part V)
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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?