Date: Sun, 20 Nov 2016 21:47:02 +0100
From: Borislav Petkov <bp@alien8.de>
To: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Matthew Whitehead <tedheadster@gmail.com>,
        Brian Gerst <brgerst@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>
Subject: Re: [PATCH] x86/boot: Fail the boot if !M486 and CPUID is missing
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On Sun, Nov 20, 2016 at 05:34:43PM -0200, Henrique de Moraes Holschuh wrote:
> On Sun, 20 Nov 2016, Borislav Petkov wrote:
> > We will have set (or not) the X86_FEATURE_CPUID bit at
> > early_identify_cpu() time. Looking at the code, we do call sync_core()
> > pretty early. :-\
> 
> Hmm, watch out for the early microcode update driver for Intel
> processors should something get changed in the implementation, or in the
> behavior of sync_core().

That's exactly what I had in mind when I wrote the above sentence...

> That driver absolutely needs to issue a cpuid (with EAX = 1) before each
> rdmsr(MSR_IA32_UCODE_REV).  And it uses sync_core() calls to do it.  A
> CR2 access just won't do in this extremely specific case.
> 
> This kind of pitfall is why I wanted to replace all use of sync_core()
> in arch/x86/kernel/cpu/microcode/intel.c with an explicit use of an
> inconditional cpuid(eax = 1)...
> 
> (note: this protocol to read MSR_IA32_UCODE_REV was made an
> architectural requirement a while ago -- it was once considered an
> erratum workaround.  It is documented in the "Intel 64 and IA‐32
> Architectures Software Developer's Manual", Volume 3A: System
> Programming Guide, Part 1, section 9.11).

Well, I'm looking at this:

"CPUID returns a value in a model specific register in addition to its
usual register return values. The semantics of CPUID cause it to deposit
an update ID value in the 64-bit model-specific register at address 08BH
(IA32_BIOS_SIGN_ID)."

And I think yes, we should do an explicit CPUID(1) regardless of
what happens to sync_core(). Feel free to send a patch against
tip:x86/microcode.

-- 
Regards/Gruss,
    Boris.

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