linux-kernel - Re: [PATCH 4/7] x86/microcode: Do not upload microcode if CPUs are offline

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Message-ID: <20180228131156.i4y6la7besdflffd@khazad-dum.debian.net>
Date:   Wed, 28 Feb 2018 10:11:56 -0300
From:   Henrique de Moraes Holschuh <hmh@....eng.br>
To:     Borislav Petkov <bp@...en8.de>
Cc:     X86 ML <x86@...nel.org>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/7] x86/microcode: Do not upload microcode if CPUs are
 offline

On Wed, 28 Feb 2018, Borislav Petkov wrote:
> Avoid loading microcode if any of the CPUs are offline, and issue a
> warning. Having different microcode revisions on the system at any time
> is outright dangerous.

Even if we update that microcode during CPU early bring-up, before we
mark it on-line and start using it?

AFAIK, late-loading or not, this is what should happen in the current
code: APs that are brought up after a microcode update is loaded (either
by the early or late driver, it doesn't matter) will be always
*early-updated* to the new microcode.

Is it dangerous to have an offline core at an older microcode revision
than the online cores?

I am not against the patch, mind you, but I am curious about why it is
supposed to be dangerous if we're updating the CPUs before we start
using them *anyway*.

Also, if this is really dangerous, does it means safe CPU hotplug isn't
possible?  AFAICT, the firmware would have to do it for us, but it
*doesn't* have the up-to-date microcode (*we* had to update it)...

-- 
  Henrique Holschuh