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

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Message-ID: <20200604133545.GB3976@hirez.programming.kicks-ass.net>
Date:   Thu, 4 Jun 2020 15:35:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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 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.