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Date:   Thu, 21 Nov 2019 18:14:01 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Fenghua Yu <fenghua.yu@intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Tony Luck <tony.luck@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter
Message-ID: <20191121171401.GE12042@gmail.com>
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* Fenghua Yu <fenghua.yu@intel.com> wrote:

> > This feature MUST be default enabled, otherwise everything will 
> > be/remain broken and we'll end up in the situation where you can't 
> > use it even if you wanted to.
> 
> The usage scope of this patch set is largely reduced to only real time. 
> The long split lock processing time (>1000 cycles) cannot be tolerated 
> by real time.
> 
> Real time customers do want to use this feature to detect the fatal 
> split lock error. They don't want any split lock issue from BIOS/EFI/ 
> firmware/kerne/drivers/user apps.
> 
> Real time can enable the feature (set bit 29 in TEST_CTRL MSR) in BIOS 
> and don't need OS to enable it. But, #AC handler cannot handle split 
> lock in the kernel and will return to the faulting instruction and 
> re-enter #AC. So current #AC handler doesn't provide useful information 
> for the customers. That's why we add the new #AC handler in this patch 
> set.

Immaterial - for this feature to be useful it must be default-enabled, 
with reasonable quirk knobs offered to people who happen to be bitten by 
such bugs and cannot fix the software.

But default-enabled is a must-have, as Peter said.

Thanks,

	Ingo