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

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Message-ID: <20180110113307.rwaxpwuvknugeoir@pd.tnic>
Date:   Wed, 10 Jan 2018 12:33:07 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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, 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.

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