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

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Message-ID: <YW766qk0W6K5rhrU@google.com>
Date:   Tue, 19 Oct 2021 17:05:46 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chenyi Qiang <chenyi.qiang@...el.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 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.

#ifndef CONFIG_X86_DEBUGCTLMSR
	if (boot_cpu_data.x86 < 6)
		return;
#endifa

	wrmsrl(MSR_IA32_DEBUGCTLMSR, 0);