linux-kernel - Re: [PATCH 6/8] x86, microcode, intel: use cpuid explicitly instead of sync

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Message-ID: <20141107175645.GC5180@pd.tnic>
Date:	Fri, 7 Nov 2014 18:56:45 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	linux-kernel@...r.kernel.org, H Peter Anvin <hpa@...or.com>
Subject: Re: [PATCH 6/8] x86, microcode, intel: use cpuid explicitly instead
 of sync_core

On Mon, Sep 08, 2014 at 02:37:52PM -0300, Henrique de Moraes Holschuh wrote:
> The protocol to safely read MSR 8BH, described in the Intel SDM vol 3A,
> section 9.11.7.1, explicitly determines that cpuid with EAX=1 must be
> used between the wrmsr(0x8B, 0); and the rdmsr(0x8B).
> 
> The microcode driver was abusing sync_core() to do this, probably
> because it predates by nearly a decade the current "asm volatile
> (:::"memory")" implementation of native_cpuid(), which is required for
> the Intel MSR 8BH access protocol.

Huh, what? Have you taken a look at sync_core() first?

> sync_core() semanthics are that of being a speculative execution
> barrier, and not "run cpuid with EAX=1".

Again, what?

Hmm, let's see:

static inline void sync_core(void)
{
 ...

        asm volatile("cpuid"
                     : "=a" (tmp)
                     : "0" (1)
                     : "ebx", "ecx", "edx", "memory");

What is the problem again?

I'm sorry but I don't understand what you're trying to fix here...

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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