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

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Message-Id: <96FD6D6D-4D56-492F-A996-F18F913F5091@amacapital.net>
Date:   Tue, 9 Jan 2018 18:57:32 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
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 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).