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Date:   Thu, 20 Aug 2020 10:35:39 +0200
From:   peterz@infradead.org
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Andy Lutomirski <luto@kernel.org>, Kyle Huey <me@kylehuey.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Robert O'Callahan <rocallahan@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
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        Brian Gerst <brgerst@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [REGRESSION 5.8] x86/entry: DR0 break-on-write not working
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On Wed, Aug 19, 2020 at 10:46:36PM -0500, Josh Poimboeuf wrote:
> On Wed, Aug 19, 2020 at 05:14:18PM -0700, Andy Lutomirski wrote:

> > I'm pretty sure you have the buggy sequence of events right, but for
> > the wrong reason.  There's nothing wrong with scheduling when
> > delivering SIGTRAP, but it's definitely wrong to blindly save and
> > restore DR7 around scheduling and around ptrace invocations.  Remember
> > this is an entry from user mode, so it runs on the user stack.

*groan*, yeah that's broken :/

> Wow, I had no idea user #DB's run on the task stack.  The scheduling
> from #DB blew my mind :-)  What's the purpose of that?

At the very least that vm86 junk needs to I think. Also -RT needs it to
send signals.