linux-kernel - Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage

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Message-ID: <20211115230737.27gqnzwvkaxoi7es@treble>
Date:   Tue, 16 Nov 2021 13:27:45 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        "linux-toolchains@...r.kernel.org" <linux-toolchains@...r.kernel.org>,
        live-patching@...r.kernel.org
Subject: Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage

On Mon, Nov 15, 2021 at 01:59:36PM +0100, Miroslav Benes wrote:
> On Fri, 12 Nov 2021, Josh Poimboeuf wrote:
> 
> > If the child schedules out, and then the parent gets patched, things can
> > go off-script if the child later jumps back to the unpatched version of
> > the parent, and then for example the old parent tries to call another
> > patched function with a since-changed ABI.
> 
> ...
>  
> > I don't know about other patch creation tooling, but I'd imagine they
> > also need to know about .cold functions, unless they have that
> > optimization disabled.  Because the func and its .cold counterpart
> > always need to be patched together.
> 
> We, at SUSE, solve the issue differently... the new patched parent would 
> call that another patched function with a changed ABI statically in a live 
> patch. So in that example, .cold child would jump back to the unpatched 
> parent which would then call, also, the unpatched function.

So if I understand correctly, if a function's ABI changes then you don't
patch it directly, but only patch its callers to call the replacement?

Is there a reason for that?

-- 
Josh