RE: [PATCH v8 13/15] x86/split_lock: Enable split lock detection by default

From: Thomas Gleixner <tglx@linutronix.de>
To: David Laight <David.Laight@ACULAB.COM>
Cc: 'Fenghua Yu' <fenghua.yu@intel.com>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	H Peter Anvin <hpa@zytor.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Ashok Raj <ashok.raj@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ravi V Shankar <ravi.v.shankar@intel.com>,
	Xiaoyao Li <xiaoyao.li@intel.com>,
	Christopherson Sean J <sean.j.christopherson@intel.com>,
	Kalle Valo <kvalo@codeaurora.org>,
	Michael Chan <michael.chan@broadcom.com>,
	linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>
Subject: RE: [PATCH v8 13/15] x86/split_lock: Enable split lock detection by default
Date: Thu, 25 Apr 2019 15:04:10 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1904251501430.1960@nanos.tec.linutronix.de> (raw)
In-Reply-To: <551673e41ac649689243cbe060364b0d@AcuMS.aculab.com>

On Thu, 25 Apr 2019, David Laight wrote:
> From: Thomas Gleixne]
> > Sent: 25 April 2019 11:59
> > On Thu, 25 Apr 2019, David Laight wrote:
> > 
> > > From:  Fenghua Yu
> > > > Sent: 24 April 2019 20:33
> > > > A split locked access locks bus and degrades overall memory access
> > > > performance. When split lock detection feature is enumerated, enable
> > > > the feature by default by writing 1 to bit 29 in MSR TEST_CTL to find
> > > > any split lock issue.
> > >
> > > You can't enable this by default until ALL the known potentially
> > > misaligned locked memory operations have been fixed.
> > 
> > Errm? The result will be a WARN_ON() printed and no further damage.
> 
> ISTR something about sending SIGSEGV to userspace.

Nope. If the #AC originates from kernel then a warning is printed and the
detection is disabled.

Userspace is a different story. We cannot audit all user space upfront,
right?

> > It's not making anything worse than it is now. In fact we just should add a
> > 
> >     WARN_ON_ONCE(!aligned_to_long(p)) to all the xxx_bit() operations.
> > 
> > so we catch them even when they do not trigger that #AC thingy.
> 
> That will explode the kernel code size.

So what? We have enough debug options which make the kernel big. One more
does not really matter.

Thanks,

	tglx