linux-kernel - Re: [PATCH v5 2/2] x86/mce: Add support for Extended Physical Address MCA changes

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Message-ID: <YlflJfyQR/j/eRkn@zn.tnic>
Date:   Thu, 14 Apr 2022 11:11:01 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Yazen Ghannam <yazen.ghannam@....com>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
        linux-edac@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, hpa@...or.com,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v5 2/2] x86/mce: Add support for Extended Physical
 Address MCA changes

On Wed, Apr 13, 2022 at 07:40:39PM +0000, Yazen Ghannam wrote:
> I think the init logic breaks here. MCE now gets enabled before clearing old
> errors. So it's possible that the old errors get overwritten by new ones.

Err, I don't understand. CR4.MCE bit description has:

"Regardless of whether machine-check exceptions are enabled, the
processor records enabled-errors when they occur."

I'm guessing enabled errors are those for which the respective bits in
the MCi_CTL banks are set. And I think the CPU comes out of reset with
those bits set.

So the overwriting will happen regardless.

The only difference here is that "[s]etting MCE to 1 enables the
machine-check exception mechanism." So you'll get a #MC raised vs
shutdown on a fatal error.

Or am I missing an angle?

> > @@ -1791,6 +1761,9 @@ static void __mcheck_cpu_check_banks(void)
> >  		if (!b->init)
> >  			continue;
> >  
> > +		wrmsrl(mca_msr_reg(i, MCA_CTL), b->ctl);
> > +		wrmsrl(mca_msr_reg(i, MCA_STATUS), 0);
> 
> Same idea here. STATUS should be cleared before turning on reporting in a bank
> using MCA_CTL.

Look at the current code. Called in this order:

__mcheck_cpu_init_clear_banks:
        wrmsrl(mca_msr_reg(i, MCA_CTL), b->ctl);
        wrmsrl(mca_msr_reg(i, MCA_STATUS), 0);
__mcheck_cpu_check_banks
	rdmsrl(mca_msr_reg(i, MCA_CTL), msrval);
        b->init = !!msrval;

STATUS *is* cleared after MCA_CTL now too.

If this ordering is wrong - and it sounds like it is - then this needs
to be a separate patch and Cc: <stable@...r.kernel.org> and needs to go
in now.

> One downside though is that the system goes longer with CR4.MCE cleared. So
> there's greater risk of encountering a shutdown due to a machine check error.

Yeah, I don't think the couple of msecs matter.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette