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

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Date:   Mon, 25 Oct 2021 14:14:28 +0800
From:   Chenyi Qiang <chenyi.qiang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] x86/bus_lock: Don't assume the init value of
 DEBUGCTLMSR.BUS_LOCK_DETECT to be zero



On 10/20/2021 1:05 AM, Sean Christopherson wrote:
> On Wed, Sep 01, 2021, Chenyi Qiang 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.
> 
> This feels like the kernel should explicitly zero out the entire MSR somewhere
> in the generic boot flow.  E.g. something like this somewhere.
> 

Yes. Meanwhile, I think kernel code prefers to explicitly set/clear the 
control bit according to the parameter. Maybe both changes should be 
applied.

> #ifndef CONFIG_X86_DEBUGCTLMSR
> 	if (boot_cpu_data.x86 < 6)
> 		return;
> #endifa
> 
> 	wrmsrl(MSR_IA32_DEBUGCTLMSR, 0);
>