linux-kernel - Re: [patch 13/16] x86/entry: Move irqflags and context tracking to C for simple idtentries

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Message-Id: <A4E714B9-ECBA-4984-986C-02B4EAF5018C@amacapital.net>
Date:   Wed, 26 Feb 2020 11:15:31 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <JGross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [patch 13/16] x86/entry: Move irqflags and context tracking to C for simple idtentries

> On Feb 26, 2020, at 8:28 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Wed, Feb 26, 2020 at 07:11:39AM -0800, Andy Lutomirski wrote:
> 
>> In some sense, this is a weakness of the magic macro approach.  Some
>> of the entries just want to have code that runs before all the entry
>> fixups.  This is an example of it.  So are the cr2 reads.  It can all
>> be made to work, but it's a bit gross.
> 
> Right. In my current pile (new patche since last posting) I also have
> one that makes #DB save-clear/restore DR7.
> 
> I got it early enough that only a watchpoint on the task stack can still
> screw us over, since I also included your patch that excludes
> cpu_entry_area.

Hmm. It would be nice to prevent watchpoints on the task stack, but that would need some trickery.  It could be done.

> 
> Pushing it earlier still would require calling into C from the entry
> stack, which I know is on your todo list, but we're not quite there yet.

Indeed.

This is my main objection to the DEFINE_IDTENTRY stuff. It’s *great* for the easy cases, but it’s not so great for the nasty cases. Maybe we should open code PF, MC, DB, etc.  (And kill the kvm special case for PF.  I have a working patch for that and I can send it.)

Anyway, this isn’t actually what I was concerned about. I meant DR6, not DR7.  Specifically, if we get traced too early in do_debug / exc_debug, we can recursively debug and clobber DR6.  The result will be incorrect behavior in the outer do_debug. We can fix this the same way we handle CR2.  I just haven’t done it on the existing entry code because it’s too messy.