linux-kernel - RE: [PATCH 3/3] x86/split_lock: Align the x86_capability array to size of unsigned long

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Date:   Wed, 18 Sep 2019 08:54:12 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Luck, Tony'" <tony.luck@...el.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        "Sai Praneeth Prakhya" <sai.praneeth.prakhya@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: RE: [PATCH 3/3] x86/split_lock: Align the x86_capability array to
 size of unsigned long


From: Luck, Tony
> Sent: 17 September 2019 20:14
> On Tue, Sep 17, 2019 at 08:29:28AM +0000, David Laight wrote:
> > From: Tony Luck
> > > Sent: 16 September 2019 23:40
> > > From: Fenghua Yu <fenghua.yu@...el.com>
> > >
> > > The x86_capability array in cpuinfo_x86 is defined as u32 and thus is
> > > naturally aligned to 4 bytes. But, set_bit() and clear_bit() require
> > > the array to be aligned to size of unsigned long (i.e. 8 bytes in
> > > 64-bit).
> > >
> > > To fix the alignment issue, align the x86_capability array to size of
> > > unsigned long by using unnamed union and 'unsigned long array_align'
> > > to force the alignment.
> > >
> > > Changing the x86_capability array's type to unsigned long may also fix
> > > the issue because the x86_capability array will be naturally aligned
> > > to size of unsigned long. But this needs additional code changes.
> > > So choose the simpler solution by setting the array's alignment to size
> > > of unsigned long.
> > >
> > > Suggested-by: David Laight <David.Laight@...lab.com>
> >
> > While this is probably the only play where this 'capabilities' array
> > has been detected as misaligned, ISTR there are several other places
> > where the identical array is defined and used.
> > These all need fixing as well.
> 
> Agree 100%  These three patches cover the places *detected* so
> far. For bisectability reasons they need to be upstream before
> the patches that add WARN_ON, or the one that turns on alignment
> traps.  As we find other places, we can fix alignments in other
> structures too.
> 
> If you remember what those other places are, please let us know
> so we can push patches to fix those.
> 
> If you have a better strategy to find them ... that also would
> be very interesting.

ISTR doing the following:
1) Looking at the other places where the x86 capabilities got stored.
2) Searching for casts of the bit functions.
Try:
grep -r --include '*.[ch]' '_bit([^(]*, *([^)]*\*)' .

	David

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