linux-kernel - Re: [PATCH v2 2/4] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access

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Date:   Fri, 29 Jun 2018 14:10:30 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 2/4] x86/split_lock: Align x86_capability to unsigned
 long to avoid split locked access

On Fri, Jun 29, 2018 at 01:48:45PM -0700, Dave Hansen wrote:
> On 06/29/2018 01:38 PM, Fenghua Yu wrote:
> > How to handle data that is used in generic code which can be used on
> > non-Intel platform? For exmple, if I do this change for struct efi in
> > include/linux/efi.h because set_bit() sets bits in efi.flags:
> > -       unsigned long flags;
> > +       unsigned long flags __aligned(unsigned long);
> >  } efi;
> > 
> > People may argue that the alignment unnecessarily increases size of 'efi'
> > on non-Intel platform which doesn't have split lock issue. Do we care this
> > argument?
> 
> Unaligned memory accesses are bad, pretty much universally.  This is a
> general good practice that we should have been doing anyway.  Let folks
> complain.  Don't let it stop you.
> 
> Also, look at the size of that structure.  Look at how many pointers it
> has.  Do you think *anyone* is going to complain about an extra 4 bytes
> in a 400-byte structure?
> 
> > Another question, there will be a bunch of one-line changes for
> > the alignment (i.e. adding __aligned(unsigned long)) in various files.
> > Will the changes be put in one big patch or in separate one-liner patches?
> 
> Just group them logically.

Sure.

Thanks.

-Fenghua