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Date: Fri, 30 Aug 2019 16:11:56 +0200
From: Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCHv2 0/4] x86/mce: protect nr_cpus from rebooting by
 broadcast mce
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To: Pingfan Liu <kernelfans@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>, Baoquan He <bhe@redhat.com>, Peter Zijlstra <peterz@infradead.org>, kexec@lists.infradead.org, Jacob Pan <jacob.jun.pan@linux.intel.com>, x86@kernel.org, Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org, Daniel Drake <drake@endlessm.com>, Ingo Molnar <mingo@redhat.com>, Qian Cai <cai@lca.pw>, Masami Hiramatsu <mhiramat@kernel.org>, Andy Lutomirski <luto@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, Gleixner <tglx@linutronix.de>, Dave Young <dyoung@redhat.com>, Vlastimil Babka <vbabka@suse.cz>, Eric Biederman <ebiederm@xmission.com>

On Tue, Aug 27, 2019 at 11:02:19AM +0800, Pingfan Liu wrote:
> v1 -> v2: fix compile warning and error on x86_32
> 
> 
> This series include two related groups:
> [1-3/4]: protect nr_cpus from rebooting by broadcast mce
> [4/4]: improve "kexec -l" robustness against broadcast mce
> 
> When I tried to fix [1], Thomas raised concern about the nr_cpus' vulnerability
> to unexpected rebooting by broadcast mce. After analysis, I think only the
> following first case suffers from the rebooting by broadcast mce. [1-3/4] aims
> to fix that issue.
> 
> *** Back ground ***
> 
> On x86 it's required to have all logical CPUs set CR4.MCE=1. Otherwise, a
> broadcast MCE observing CR4.MCE=0b on any core will shutdown the machine.
> 
> The option 'nosmt' has already complied with the above rule by Thomas's patch.
> For detail, refer to 506a66f3748 (Revert "x86/apic: Ignore secondary threads if
> nosmt=force")
> 
> But for nr_cpus option, the exposure to broadcast MCE is a little complicated,
> and can be categorized into three cases.

One thing is not clear to me: are you "fixing" a hypothetical case here
or have you *actually* experienced an MCE happening while kdumping with
nr_cpus < num_online_cpus()?

Btw, pls do not use lkml.org to refer to previous mails but

http://lkml.kernel.org/r/<Message-ID>

Thx.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

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