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Date:   Wed, 16 Dec 2020 09:40:59 +0100
From:   Peter Zijlstra <>
To:     Josh Poimboeuf <>
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 Tue, Dec 15, 2020 at 06:38:02PM -0600, Josh Poimboeuf wrote:
> On Tue, Dec 15, 2020 at 03:54:08PM +0100, Peter Zijlstra wrote:
> > The problem is that a single instance of unwind information (ORC) must
> > capture and correctly unwind all alternatives. Since the trivially
> > correct mandate is out, implement the straight forward brute-force
> > approach:
> > 
> >  1) generate CFI information for each alternative
> > 
> >  2) unwind every alternative with the merge-sort of the previously
> >     generated CFI information -- O(n^2)
> > 
> >  3) for any possible conflict: yell.
> > 
> >  4) Generate ORC with merge-sort
> > 
> > Specifically for 3 there are two possible classes of conflicts:
> > 
> >  - the merge-sort itself could find conflicting CFI for the same
> >    offset.
> > 
> >  - the unwind can fail with the merged CFI.
> So much algorithm.


It's not really hard, but it has a few pesky details (as always).

> 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.

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.

> [ 'offset' is a byte offset from the beginning of the group.  It could
>   be calculated based on 'orig_insn' or 'orig_insn->alts', depending on
>   whether 'insn' is an original or a replacement. ]

That's exactly what it already does ofcourse ;-)

> If the array entry is NULL, just update it with a pointer to the CFI.
> If it's not NULL, make sure it matches the existing CFI, and WARN if it
> doesn't.
> Also, with this data structure, the ORC generation should also be a lot
> more straightforward, just ignore the NULL entries.

Yeah, I suppose it gets rid of the memcmp-prev thing.

> 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.

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