linux-kernel - Re: [PATCHv2 0/4] x86/mce: protect nr_cpus from rebooting by broadcast mce

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Message-ID: <20190830141156.GB30413@zn.tnic>
Date:   Fri, 30 Aug 2019 16:11:56 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     Gleixner <tglx@...utronix.de>, Andy Lutomirski <luto@...nel.org>,
        x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, Qian Cai <cai@....pw>,
        Vlastimil Babka <vbabka@...e.cz>,
        Daniel Drake <drake@...lessm.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, Dave Young <dyoung@...hat.com>,
        Baoquan He <bhe@...hat.com>, kexec@...ts.infradead.org
Subject: Re: [PATCHv2 0/4] x86/mce: protect nr_cpus from rebooting by
 broadcast mce

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.