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Date:   Mon, 4 Mar 2019 21:08:09 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Fenghua Yu <fenghua.yu@intel.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Xiaoyao Li <xiaoyao.li@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, kvm@vger.kernel.org
Subject: Re: [PATCH v4 04/17] x86/split_lock: Align x86_capability to
 unsigned long to avoid split locked access
Message-ID: <20190304200809.GS32494@hirez.programming.kicks-ass.net>
References: <1551494711-213533-1-git-send-email-fenghua.yu@intel.com>
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 <b9a7e3a6-d89c-9ce7-8909-e09122c93f96@intel.com>
 <20190304191512.GB231946@romley-ivt3.sc.intel.com>
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On Mon, Mar 04, 2019 at 11:15:12AM -0800, Fenghua Yu wrote:
> On Mon, Mar 04, 2019 at 10:52:19AM -0800, Dave Hansen wrote:
> > On 3/1/19 6:44 PM, Fenghua Yu wrote:
> > > diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> > > index 33051436c864..eb8ae701ef65 100644
> > > --- a/arch/x86/include/asm/processor.h
> > > +++ b/arch/x86/include/asm/processor.h
> > > @@ -93,7 +93,9 @@ struct cpuinfo_x86 {
> > >  	__u32			extended_cpuid_level;
> > >  	/* Maximum supported CPUID level, -1=no CPUID: */
> > >  	int			cpuid_level;
> > > -	__u32			x86_capability[NCAPINTS + NBUGINTS];
> > > +	/* Unsigned long alignment to avoid split lock in atomic bitmap ops */
> > > +	__u32			x86_capability[NCAPINTS + NBUGINTS]
> > > +				__aligned(sizeof(unsigned long));
> > 
> > I think this also warrants a comment in the changelog about the
> > alignment of 'struct cpuinfo_x86'.
> 
> How about add "Depending on the starting address where GCC generates
> for data of struct cpuinfo_x86, x86_capability[] may or may not align to
> unsigned long...."?

Composite types inherit the strictest alignment of their members.
Without that alignment hints would be utterly useless.