linux-kernel - Re: [PATCH v2 1/2] x86/mce/AMD: Redo use of SMCA MCA

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Date:   Wed, 5 Apr 2017 20:22:23 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Ghannam, Yazen" <Yazen.Ghannam@....com>
Cc:     "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        Tony Luck <tony.luck@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] x86/mce/AMD: Redo use of SMCA MCA_DE{STAT,ADDR}
 registers

On Wed, Apr 05, 2017 at 05:53:57PM +0000, Ghannam, Yazen wrote:
> Correct, but only on SMCA systems.

On !SMCA systems you log only once anyway - you don't have DE* MSRs.

> This works so I don't know why it's not okay.

So I should take the code just because you have found one way that it
works? Screw readability or future proof design - people should dump the
code on maintainers and maintainers should deal with the stinking pile.
Who cares, it works for you.

> Your suggestion also does an SMCA check.

Why TF does that even matter? Enough with the dumb checks argument
already.

> So code that does a check-and-return is preferable to code using
> if/else-if statements? If that's the case then I can try to rework it.

No, readable code matters. Your suggestion is confusing. I see

	if (is_deferred_error(status)) {
		__log_error
	} else if (mce_flags.smca) {
		...
		if (is_deferred_error(status))

But then here --->

we still deal with deferred errors. Even though we did the deferred
check in the if-clause. And that is confusing and makes the code hard to
follow.

My suggestion does *exactly* what the commit message says:

	if (__log_error(..))
		goto out;

when we go to the out label, we know we've succeeded logging the
deferred error and we're done.

        if (!mce_flags.smca)
                return;

When we return here, we know, we've taken care of the !SMCA systems. Now
here starts the second attempt to read the DE* registers and we *know*
we're on SMCA systems.

> How does log_error() know if we can't use the normal MSRs?

MCI_STATUS_VAL.

> We check for MCI_STATUS_VAL in log_error().

Yes.

> We also need to check for MCI_STATUS_DEFERRED but only if we're coming
> from the deferred error handler.

Why? We *are* coming from the #DF handler so are you expecting a
different type of error in the MSRs?

-- 
Regards/Gruss,
    Boris.

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