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Subject: Re: [PATCH 03/24] x86/paravirt: Annotate indirect calls
To: Peter Zijlstra <peterz@infradead.org>,
 David Woodhouse <dwmw2@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Josh Poimboeuf
 <jpoimboe@redhat.com>, linux-kernel@vger.kernel.org,
 Dave Hansen <dave.hansen@intel.com>, Ashok Raj <ashok.raj@intel.com>,
 Tim Chen <tim.c.chen@linux.intel.com>, Andy Lutomirski <luto@kernel.org>,
 Linus Torvalds <torvalds@linux-foundation.org>,
 Greg KH <gregkh@linuxfoundation.org>, Andrea Arcangeli
 <aarcange@redhat.com>, Andi Kleen <ak@linux.intel.com>,
 Arjan Van De Ven <arjan.van.de.ven@intel.com>,
 Dan Williams <dan.j.williams@intel.com>, Paolo Bonzini
 <pbonzini@redhat.com>, Jun Nakajima <jun.nakajima@intel.com>,
 Asit Mallick <asit.k.mallick@intel.com>, Jason Baron <jbaron@akamai.com>
References: <20180123152539.374360046@infradead.org>
 <20180123152638.162540737@infradead.org>
 <1516874525.30244.41.camel@infradead.org>
 <20180125102253.GB2228@hirez.programming.kicks-ass.net>
From: Juergen Gross <jgross@suse.com>
Message-ID: <81ade568-6ab3-a62a-5026-5138f2313ab3@suse.com>
Date: Thu, 25 Jan 2018 11:26:57 +0100
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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