Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter

From: Ingo Molnar
Date: Thu Nov 21 2019 - 12:14:09 EST

* Fenghua Yu <fenghua.yu@xxxxxxxxx> 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.