linux-kernel - Re: [patch RFC 1/5] x86/CPU: Sync CPU feature flags late

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Date:   Wed, 10 Jan 2018 13:38:35 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
cc:     Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Borislav Petkov <bp@...e.de>,
        "Raj, Ashok" <ashok.raj@...el.com>
Subject: Re: [patch RFC 1/5] x86/CPU: Sync CPU feature flags late

On Wed, 10 Jan 2018, Borislav Petkov wrote:
> 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.

/me exposes it to the flame-thrower