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

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Message-ID: <81ade568-6ab3-a62a-5026-5138f2313ab3@suse.com>
Date:   Thu, 25 Jan 2018 11:26:57 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        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 25/01/18 11:22, Peter Zijlstra wrote:
> 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.
> 

paravirt_patch_default() is the function you want to look at: it either
replaces the indirect call by some cutom code (which is never an
indirect call) or by a call of the target function.


Juergen