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

Borislav Petkov
2022-04-14T09:11:44Z

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@vger.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.