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

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Message-ID: <alpine.DEB.2.20.1801101202000.1919@nanos>
Date:   Wed, 10 Jan 2018 12:02:54 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>
cc:     "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        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 Tue, 9 Jan 2018, Andy Lutomirski wrote:
> > On Jan 9, 2018, at 5:47 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > 
> > On Wed, 10 Jan 2018, Van De Ven, Arjan wrote:
> >>> In other words, if you use late microcode loading for getting IBRS, you
> >>> don't get ALTERNATIVE patching and its benefits?
> >>> 
> >>> I'll also profess some microcode ignorance here.  Is "late microcode
> >>> patching" *all* of the stuff we do from the OS, or do we have early and
> >>> late Linux loading in addition to what the BIOS can do?
> >> 
> >> the early boot loader level stuff is much better generally (but does not
> >> work when the microcode comes out after the system booted... like really
> >> long uptimes)
> > 
> > That stuff indeed would be way simpler w/o the late support, but the fact
> > that the microcode for this might reach the user way later than the kernel
> > support makes it almost a must to support the late loading.
> 

> How hard would it be to add a late alternative feature?  Concretely, we'd
> have a list of "late" cpufeatures.  When we scan the alternative list, if
> we find a late feature, we copy it to some other list that isn't
> discarded, and we also copy its replacement (and relocate it eagerly,
> since we'll lose the offset).

It shouldn't be rocket science, but that's not for now. And yes, we should
do it.

Thanks,

	tglx