linux-kernel - Re: [PATCH v2 10/12] x86,nospec: Simplify {JMP,CALL}

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Message-ID: <20241116093901.GK22801@noisy.programming.kicks-ass.net>
Date: Sat, 16 Nov 2024 10:39:01 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: seanjc@...gle.com, pbonzini@...hat.com, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
	jthoughton@...gle.com
Subject: Re: [PATCH v2 10/12] x86,nospec: Simplify {JMP,CALL}_NOSPEC (part 2)

On Fri, Nov 15, 2024 at 10:40:56AM -0800, Josh Poimboeuf wrote:
> On Mon, Nov 11, 2024 at 12:59:45PM +0100, Peter Zijlstra wrote:
> > Counterpart to 09d09531a51a ("x86,nospec: Simplify
> > {JMP,CALL}_NOSPEC"), x86_64 will rewrite all this anyway, see
> > apply_retpoline.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  arch/x86/include/asm/nospec-branch.h |   29 +++++++++++------------------
> >  1 file changed, 11 insertions(+), 18 deletions(-)
> > 
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -429,31 +429,24 @@ static inline void call_depth_return_thu
> >  
> >  #ifdef CONFIG_X86_64
> >  
> > +#define __CS_PREFIX						\
> > +	".irp rs,r8,r9,r10,r11,r12,r13,r14,r15\n"		\
> > +	".ifc %V[thunk_target],\\rs\n"				\
> > +	".byte 0x2e\n"						\
> > +	".endif\n"						\
> > +	".endr\n"
> 
> After staring at this for some minutes I'm thinking it would be helpful
> to add a comment saying this is equivalent to
> -mindirect-branch-cs-prefix.

I'll just copy-paste the comment from the other __CS_PREFIX elsewhere in
this file :-)