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Date:   Thu, 4 Jun 2020 15:35:45 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     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>
Subject: Re: [patch V9 00/39] x86/entry: Rework leftovers (was part V)
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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.