linux-kernel - Re: [PATCH v2 00/12] x86: major paravirt cleanup

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Message-ID: <20201216165605.4h5q7os5dutjgdqi@treble>
Date:   Wed, 16 Dec 2020 10:56:05 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jürgen Groß <jgross@...e.com>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-hyperv@...r.kernel.org, kvm@...r.kernel.org, luto@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Deep Shah <sdeep@...are.com>,
        "VMware, Inc." <pv-drivers@...are.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
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 ;-)

-- 
Josh