linux-kernel - Re: [PATCH 1/6] x86/entry: Introduce local_db

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Message-ID: <20200529190218.GP706495@hirez.programming.kicks-ass.net>
Date:   Fri, 29 May 2020 21:02:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        sean.j.christopherson@...el.com, daniel.thompson@...aro.org
Subject: Re: [PATCH 1/6] x86/entry: Introduce local_db_{save,restore}()

On Fri, May 29, 2020 at 10:28:33AM -0700, Andy Lutomirski wrote:
> > +static __always_inline unsigned long local_db_save(void)
> > +{
> > +    unsigned long dr7;
> > +
> > +    get_debugreg(&dr7, 7);
> > +    dr7 ^= 0x400;
> 
> Why xor?  This seems extra confusing.

I'll do the normal mask thing ..