linux-kernel - Re: [PATCH 03/24] x86/paravirt: Annotate indirect calls

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Message-ID: <20180125102253.GB2228@hirez.programming.kicks-ass.net>
Date:   Thu, 25 Jan 2018 11:22:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH 03/24] x86/paravirt: Annotate indirect calls

On Thu, Jan 25, 2018 at 10:02:05AM +0000, David Woodhouse wrote:
> On Tue, 2018-01-23 at 16:25 +0100, Peter Zijlstra wrote:
> > Paravirt emits indirect calls which get flagged by objtool retpoline
> > checks, annotate it away because all these indirect calls will be
> > patched out before we start userspace.
> 
> I've seen this asserted repeatedly but I've never truly convinced
> myself of it. Is this absolutely unconditionally true in every case,
> even when we're running as a guest and there are *actual* calls to be
> made? We turn them into direct calls, never leave them indirect?

That is my understanding; and when I worked on the paravirt spinlock
code and disassembled live guest code this seemed to have happend.

But let me go read the paravirt code again to make a stronger argument
in favour.