linux-kernel - Re: [PATCH 2/3 V2] x86/mce: Fix incorrect "Machine check from unknown source" message

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Date:   Tue, 19 Jun 2018 12:30:17 +0200
From:   Borislav Petkov <bp@...e.de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     "Williams, Dan J" <dan.j.williams@...el.com>,
        "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3 V2] x86/mce: Fix incorrect "Machine check from
 unknown source" message

On Wed, May 30, 2018 at 11:26:32AM +0200, Borislav Petkov wrote:
> > In "x86/mce: Exit properly when no banks to poll" you
> > leap right to the end.  I'm wondering whether this can
> > ever happen? I mean, if there are no machine check banks,
> > then how did we get a machine check?
> 
> Right, so this looks like some remnant from old times, lemme do some
> archeology...
> 
> /me goes and dusts off the full history linux repo...
> 
> I found this:
> 
> commit 7dd1e1d805d15ca63d05badf40026629ba75cbc8
> Author: Andi Kleen <ak@...e.de>
> Date:   Tue Feb 24 17:58:41 2004 -0800
> 
>     [PATCH] New machine check handler for x86-64
> 
> and there's no mention why the !banks check is there.
> 
> I'm wondering if we should simply remove it. I mean, as you say, if
> there are no MCA banks, we won't be running in here in the first
> place...
> 
> > Both the original, and your new code, skip the:
> > 
> > 	mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
> > 
> > which seems bad. That leaves MCG_STATUS.MCIP set ... so a second
> > machine check would just reset the machine.
> 
> That's a good point. It goes away as an issue if we simply drop the
> check.

...and gone it is.

---
From: Borislav Petkov <bp@...e.de>
Date: Tue, 19 Jun 2018 12:27:02 +0200
Subject: [PATCH] x86/mce: Remove !banks check

If we don't have MCA banks, we won't see machine checks anyway. Drop the
check.

Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/kernel/cpu/mcheck/mce.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index ea1521ec7e5b..05d2d15a95bd 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -1185,9 +1185,6 @@ void do_machine_check(struct pt_regs *regs, long error_code)
 
 	this_cpu_inc(mce_exception_count);
 
-	if (!cfg->banks)
-		goto out;
-
 	mce_gather_info(&m, regs);
 	m.tsc = rdtsc();
 
@@ -1327,7 +1324,7 @@ void do_machine_check(struct pt_regs *regs, long error_code)
 	if (worst > 0)
 		mce_report_event(regs);
 	mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
-out:
+
 	sync_core();
 
 	if (worst != MCE_AR_SEVERITY && !kill_it)
-- 
2.17.0.582.gccdcbd54c

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