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Date:   Wed, 16 Dec 2020 10:56:05 -0600
From:   Josh Poimboeuf <>
To:     Peter Zijlstra <>
Cc:     Jürgen Groß <>,,,,,,,,
        Thomas Gleixner <>,
        Ingo Molnar <>, Borislav Petkov <>,
        "H. Peter Anvin" <>,
        Boris Ostrovsky <>,
        Stefano Stabellini <>,
        Deep Shah <>,
        "VMware, Inc." <>,
        "K. Y. Srinivasan" <>,
        Haiyang Zhang <>,
        Stephen Hemminger <>,
        Wei Liu <>,
        Paolo Bonzini <>,
        Sean Christopherson <>,
        Vitaly Kuznetsov <>,
        Wanpeng Li <>,
        Jim Mattson <>,
        Joerg Roedel <>,
        Daniel Lezcano <>,
        Juri Lelli <>,
        Vincent Guittot <>,
        Dietmar Eggemann <>,
        Steven Rostedt <>,
        Ben Segall <>, Mel Gorman <>,
        Daniel Bristot de Oliveira <>
Subject: Re: [PATCH v2 00/12] x86: major paravirt cleanup

On Wed, Dec 16, 2020 at 09:40:59AM +0100, Peter Zijlstra wrote:
> > So much algorithm.
> :-)
> It's not really hard, but it has a few pesky details (as always).

It really hurt my brain to look at it.

> > Could we make it easier by caching the shared
> > per-alt-group CFI state somewhere along the way?
> Yes, but when I tried it grew the code required. Runtime costs would be
> less, but I figured that since alternatives are typically few and small,
> that wasn't a real consideration.

Aren't alternatives going to be everywhere now with paravirt using them?

> That is, it would basically cache the results of find_alt_unwind(), but
> you still need find_alt_unwind() to generate that data, and so you gain
> the code for filling and using the extra data structure.
> Yes, computing it 3 times is naf, but meh.

Haha, I loved this sentence.

> > Thoughts?  This is all theoretical of course, I could try to do a patch
> > tomorrow.
> No real objection, I just didn't do it because 1) it works, and 2) even
> moar lines.

I'm kind of surprised it would need moar lines.  Let me play around with
it and maybe I'll come around ;-)


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