netdev - RE: [PATCH v7 04/21] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access

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Message-ID: <aceeddaa107a4aed9add47945acaa129@AcuMS.aculab.com>
Date:   Thu, 18 Apr 2019 13:14:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Thomas Gleixner' <tglx@...utronix.de>
CC:     'Fenghua Yu' <fenghua.yu@...el.com>,
        'Ingo Molnar' <mingo@...hat.com>,
        'Borislav Petkov' <bp@...en8.de>,
        'H Peter Anvin' <hpa@...or.com>,
        "'Paolo Bonzini'" <pbonzini@...hat.com>,
        'Dave Hansen' <dave.hansen@...el.com>,
        'Ashok Raj' <ashok.raj@...el.com>,
        'Peter Zijlstra' <peterz@...radead.org>,
        'Ravi V Shankar' <ravi.v.shankar@...el.com>,
        'Xiaoyao Li ' <xiaoyao.li@...el.com>,
        'Christopherson Sean J' <sean.j.christopherson@...el.com>,
        'Kalle Valo' <kvalo@...eaurora.org>,
        'Michael Chan' <michael.chan@...adcom.com>,
        'linux-kernel' <linux-kernel@...r.kernel.org>,
        'x86' <x86@...nel.org>,
        "'kvm@...r.kernel.org'" <kvm@...r.kernel.org>,
        "'netdev@...r.kernel.org'" <netdev@...r.kernel.org>,
        "'linux-wireless@...r.kernel.org'" <linux-wireless@...r.kernel.org>
Subject: RE: [PATCH v7 04/21] x86/split_lock: Align x86_capability to unsigned
 long to avoid split locked access

From: Thomas Gleixner
> Sent: 18 April 2019 12:49
> On Thu, 18 Apr 2019, David Laight wrote:
> > From: David Laight
> > > Sent: 18 April 2019 10:21
> > > From: Fenghua Yu
> > > > Sent: 17 April 2019 22:34
> > > >
> > > > set_cpu_cap() calls locked BTS and clear_cpu_cap() calls locked BTR to
> > > > operate on bitmap defined in x86_capability.
> > > >
> > > > Locked BTS/BTR accesses a single unsigned long location. In 64-bit mode,
> > > > the location is at:
> > > > base address of x86_capability + (bit offset in x86_capability / 64) * 8
> > > >
> > > > Since base address of x86_capability may not be aligned to unsigned long,
> > > > the single unsigned long location may cross two cache lines and
> > > > accessing the location by locked BTS/BTR introductions will cause
> > > > split lock.
> > >
> > > Isn't the problem that the type (and definition) of x86_capability[] are wrong.
> > > If the 'bitmap' functions are used for it, it should be defined as a bitmap.
> > > This would make it 'unsigned long' not __u32.
> > >
> > > This type munging of bitmaps only works on LE systems.
> > >
> > > OTOH the locked BTS/BTR instructions could be changed to use 32 bit accesses.
> > > ISTR some of the associated functions use byte accesses.
> > >
> > > Perhaps there ought to be asm wrappers for BTS/BTR that do 8bit and
> > > 32bit accesses.
> >
> > A quick look shows that this isn't the only __32[] that is being
> > cast to (unsigned long) and then to set/test/clear_bit() in those
> > files.
> >
> > I wonder how much other code is applying such casts?
> 
> The reason for the cpuid stuff using u32 is that this is actually the width
> of the information retrieved from CPUID.

Right, but you shouldn't (as has been found out) cast pointers
to integer types.

Running
grep -r --include '*.[ch]' '_bit([^(]*, *([^)]* ' .
over the entire kernel source tree shows quite a few 'dubious' casts.

They'll be doubly dubious on BE systems.

	David

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