linux-kernel - Re: [RESEND] x86/bus_lock: Don't assume the init value of DEBUGCTLMSR.BUS_LOCK

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Date:   Tue, 2 Aug 2022 13:42:45 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Chenyi Qiang <chenyi.qiang@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [RESEND] x86/bus_lock: Don't assume the init value of
 DEBUGCTLMSR.BUS_LOCK_DETECT to be zero


* Chenyi Qiang <chenyi.qiang@...el.com> wrote:

> 
> 
> On 8/2/2022 6:51 PM, Ingo Molnar wrote:
> > 
> > * Chenyi Qiang <chenyi.qiang@...el.com> wrote:
> > 
> > > It's possible that BIOS/firmware has set DEBUGCTLMSR_BUS_LOCK_DETECT, or
> > > this kernel has been kexec'd from a kernel that enabled bus lock
> > > detection.
> > > 
> > > Disable bus lock detection explicitly if not wanted.
> > 
> > Makes sense.
> > 
> > Just curious: in what circumstances does the BIOS/firmware set
> > DEBUGCTLMSR_BUS_LOCK_DETECT? Does it use it, or does it enable it for some
> > spurious reason, without really using the feature? (Assuming you are aware
> > of instances where this happened - or was this simply a hypothetical?)
> 
> Yes, It's just a hypothetical for BIOS/firmware. Kexec is the real case I
> met with this problem.

Fair enough, I've tweaked the changelog a bit to de-emphasize the firmware 
angle, and applied your fix to tip:x86/urgent.

Thanks,

	Ingo