linux-kernel - Re: [RFC][PATCH 0/4] x86/entry: disallow #DB more

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Message-ID: <20200525104038.GC317569@hirez.programming.kicks-ass.net>
Date:   Mon, 25 May 2020 12:40:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>
Subject: Re: [RFC][PATCH 0/4] x86/entry: disallow #DB more

On Mon, May 25, 2020 at 12:02:48PM +0200, Rasmus Villemoes wrote:

> Naive question: did you check disassembly to see whether gcc threw your
> native_get_debugreg() away, given that the asm isn't volatile and the
> result is not used for anything? Testing here only shows a "mov
> %r9,%db7", but the read did seem to get thrown away.

Argh.. no I did not. Writing it all in asm gets me:

[    1.627405] XXX: 3900 8304 22632

which is a lot worse...