linux-kernel - Re: 3.2-rc2 freezes on boot for AMD K6 - bisected to commit bcb80e53877c2045d9e52f4a71372c3fe6501f6f

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Date:	Sat, 3 Dec 2011 15:07:56 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Larry Finger <Larry.Finger@...inger.net>,
	Ingo Molnar <mingo@...e.hu>
Cc:	Borislav Petkov <borislav.petkov@....com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: 3.2-rc2 freezes on boot for AMD K6 - bisected to commit bcb80e53877c2045d9e52f4a71372c3fe6501f6f

On Sat, Dec 3, 2011 at 12:43 PM, Larry Finger <Larry.Finger@...inger.net> wrote:
> On 11/30/2011 01:09 AM, Larry Finger wrote:
>> On 11/29/2011 11:59 PM, Srivatsa S. Bhat wrote:
>>>
>>> Can you please try out the patch posted in
>>> https://lkml.org/lkml/2011/11/28/178 ?

Ugh. I hate that patch.

It's completely stupid. If "rdmsr_safe()" doesn't work at that point
in the boot, then it's pointless to call it.

So this change is pure and utter crap:

  -	rdmsr_safe(MSR_AMD64_PATCH_LEVEL, &c->microcode, &dummy);
  +	if (c->x86 >= 0xf)
  +		rdmsr_safe(MSR_AMD64_PATCH_LEVEL, &c->microcode, &dummy);

because it is misleading as hell: that rdmsr isn't *safe* at all, so
why are we calling "rdmsr_safe()"?

It's wrong.

The right patch would either just remove the "safe" part (and just say
that the register has to be supported if c->x86 >= 0xf), but quite
honestly, I don't see why we do that thing in early_init_amd() AT ALL.
Afaik, the microcode version field isn't really *needed* by the
kernelin the first place, much less is it needed by the *early* boot,
so why isn't this in 'init_amd()' a bit later when the "safe" version
actually *works*?

IOW, I think the patch should be something like the attached (TOTALLY
UNTESTED) patch. Larry, does this work for you? It just moves the
rdmsr_safe() to the later function.

Borislav?

> I just updated mainline to 3.2-rc4, and that patch is not included. Please
> check with Ingo to see why it was not available. It is a real show stopper
> for old AMD CPUs.

Ingo seems to have fallen off the earth for the last two weeks.
There's *one* email form him about 12 hours ago, before that the last
one I see is from early November.

Ingo, everything ok?

                   Linus

View attachment "patch.diff" of type "text/x-patch" (1005 bytes)