From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752316AbaAYQgF (ORCPT ); Sat, 25 Jan 2014 11:36:05 -0500 Received: from out4-smtp.messagingengine.com ([66.111.4.28]:40343 "EHLO out4-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751934AbaAYQgD (ORCPT ); Sat, 25 Jan 2014 11:36:03 -0500 X-Sasl-enc: fNXipq5KNIaoLF86xAgZxbBk1vHb7EEfB8XFZJAQ43rl 1390667762 Date: Sat, 25 Jan 2014 14:35:58 -0200 From: Henrique de Moraes Holschuh To: Andi Kleen Cc: x86@kernel.org, linux-kernel@vger.kernel.org, Andi Kleen Subject: Re: [PATCH 2/2] x86, microcode: Add option to allow downgrading of microcode Message-ID: <20140125163558.GC25531@khazad-dum.debian.net> References: <1390598339-18740-1-git-send-email-andi@firstfloor.org> <1390598339-18740-2-git-send-email-andi@firstfloor.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1390598339-18740-2-git-send-email-andi@firstfloor.org> X-GPG-Fingerprint1: 4096R/39CB4807 C467 A717 507B BAFE D3C1 6092 0BD9 E811 39CB 4807 X-GPG-Fingerprint2: 1024D/1CDB0FE3 5422 5C61 F6B7 06FB 7E04 3738 EE25 DE3F 1CDB 0FE3 User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 24 Jan 2014, Andi Kleen wrote: > For testing purposes it can be useful to downgrade microcode. > Normally the driver only allows upgrading. The code is not prepared to work correctly when downgrading is allowed, in the presence of shadowed microcode. When a firmware request results in more than one microcode for the same cpuid, with overlapping pf_mask, the code *depends* on the "never downgrade" logic to work. Shadowed microcode *is* currently distributed by Intel, and as an artifact of the f-m-s grouping, it is *guaranteed* to trigger the issue. When the issue triggers, what microcode will be selected to be uploaded to the core depends *only* on the order of the microcodes in the firmware file. I see no documentation of this fact anywhere, and it is *anything* but obvious. That extremely obnoxious Intel microcode license forbids anyone to fix the pf_mask metadata fields to remove shadowing, so we ship that stuff as-is to in the distros. It *will* hit users. Also, since you're going to mess with this, why don't you implement the correct semanthics for microcode with the sign bit set? Making it signed actually makes the current code behaviour worse. Refer to: http://lkml.org/lkml/2011/3/21/522 Note that I was wrong about negative revision microcode not being found in the wild. Intel has shipped entry-level server boards with pre-release microcode several times, even on BIOS updates, and you're likely to get access to such pre-release microcode if you're dealing with Intel firmware partners that has full access to microcode updates. -- "One disk to rule them all, One disk to find them. One disk to bring them all and in the darkness grind them. In the Land of Redmond where the shadows lie." -- The Silicon Valley Tarot Henrique Holschuh