Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755052AbeAJLdU (ORCPT + 1 other); Wed, 10 Jan 2018 06:33:20 -0500 Received: from mail.skyhub.de ([5.9.137.197]:55402 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753970AbeAJLdS (ORCPT ); Wed, 10 Jan 2018 06:33:18 -0500 Date: Wed, 10 Jan 2018 12:33:07 +0100 From: Borislav Petkov To: Ingo Molnar Cc: Dave Hansen , Thomas Gleixner , LKML , Linus Torvalds , x86@kernel.org, Peter Zijlstra , David Woodhouse , Tim Chen , Andrea Arcangeli , Andi Kleen , Greg KH , Andy Lutomirski , Arjan Van De Ven , Borislav Petkov , "Raj, Ashok" Subject: Re: [patch RFC 1/5] x86/CPU: Sync CPU feature flags late Message-ID: <20180110113307.rwaxpwuvknugeoir@pd.tnic> References: <20180110010652.404145126@linutronix.de> <20180110011350.501418723@linutronix.de> <20180110062013.47tbgtccf2wgp5td@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20180110062013.47tbgtccf2wgp5td@gmail.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: On Wed, Jan 10, 2018 at 07:20:13AM +0100, Ingo Molnar wrote: > it be really unreasonable to say that if a microcode update changes CPU > flags an initrd rebuild and a reboot is required? It's not like microcode updates > are _that_ frequent - in fact they tend to be much _less_ frequent in a system's > life time than kernel updates. > > So all of this 'late loading' and CPU flag splitting complexity seems unnecessary > to me: we should be glad we do early microcode loading now, and should embrace it. > > Changing CPU features way after the CPU has booted up is possible, and we could in > theory extend code patching to work 'late' as well, but given how infrequent all > this is bound to be in practice I fear it's all going to be a big, seldom tested, > often broken mess, with no real benefit to users. Agreed: we support that late patching for those use cases where machines run for a long time, simulating all kinds of crap. And frankly, if those things need to get IBRS all of a sudden and *not* reboot, then something's wrong with the whole contraption setup. So yes, I'd vote too for supporting only early IBRS and not do the late thing now. Maybe later, if there's, like, a really compelling use case. I will have to do the late thing for our old kernels which don't have early loading but that would be a one-off and my problem. -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.