linux-kernel - Re: [PATCH 1/1] x86/microcode: vsnprintf() might be unavailable

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Message-ID: <20150602170040.GE20576@pd.tnic>
Date:	Tue, 2 Jun 2015 19:00:40 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH 1/1] x86/microcode: vsnprintf() might be unavailable

On Tue, Jun 02, 2015 at 07:49:53PM +0300, Andy Shevchenko wrote:
> Initial paging setup is involved?

Yes, load_ucode_bsp happens before paging is enabled on 32-bit. And
that should remain that way - we want microcode application as early as
possible.

> Hmm… which Intel CPUs you run on?

processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 58
model name      : Intel(R) Core(TM) i7-3520M CPU @ 2.90GHz
stepping        : 9
microcode       : 0x1b

and an SNB one too.

> Ah, one more thing we run our kernel from kexec (I hope it's not a
> case, but who knows).

Does it work when you boot a normal 32-bit kernel with builtin microcode
on it?

In order to load built-in microcode, you'll have to enable the boot
options in the commit message to

  760d765b2bb6 ("x86/microcode: Parse built-in microcode early")

Let me know if something's not clear.

On Tue, Jun 02, 2015 at 07:46:29PM +0300, Andy Shevchenko wrote:
> > @@ -533,7 +533,12 @@ static bool __init load_builtin_intel_microcode(struct cpio_data *cp)
> >  	model    = x86_model(eax);
> >  	stepping = eax & 0xf;
> >  
> > -	sprintf(name, "intel-ucode/%02x-%02x-%02x", family, model, stepping);
> > +	*p++ = '/';
> > +	p = hex_byte_pack(p, family);
> > +	*p++ = '-';
> > +	p = hex_byte_pack(p, model);
> > +	*p++ = '-';
> > +	p = hex_byte_pack(p, stepping);
> >  
> 
> Forgot to add *p = '\0'; here, but it doesn't really mater for the idea.
> 
> What I would like to tell that if I move 'call load_ucode_bsp' after
> paging is done, it seems sprintf() starts working nicely.

Hmm, ok, so something's not able to stomach pre-paging. Can you dump
something from the failure, RIP, stack, whatever?

Can I reproduce it?

> So, I don't know which fix is better (or neither),

See above.

>  and wondering if we need to fix AMD part.

No need, AFAICT. It did work in my testing.

> P.S. It would be really nice to get an expert / detailed explanation
> what is going on there.

Yeah, that's why I'm asking.

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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