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Date: Sun, 20 Nov 2016 17:34:43 -0200
From: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
To: Borislav Petkov <bp@alien8.de>
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
Message-ID: <20161120193442.GA1145@khazad-dum.debian.net>
References: <70eac6639f23df8be5fe03fa1984aedd5d40077a.1479598603.git.luto@kernel.org>
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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 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).

-- 
  Henrique Holschuh