linux-kernel - Re: [RFC][PATCH 05/17] x86/cpu: Cleanup the untrain mess

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Date:   Wed, 9 Aug 2023 15:12:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, David.Kaplan@....com,
        Andrew.Cooper3@...rix.com, gregkh@...uxfoundation.org
Subject: Re: [RFC][PATCH 05/17] x86/cpu: Cleanup the untrain mess

On Wed, Aug 09, 2023 at 08:51:01AM -0400, Josh Poimboeuf wrote:
> On Wed, Aug 09, 2023 at 09:12:23AM +0200, Peter Zijlstra wrote:
> > Since there can only be one active return_thunk, there only needs be
> > one (matching) untrain_ret. It fundamentally doesn't make sense to
> > allow multiple untrain_ret at the same time.
> > 
> > Fold all the 3 different untrain methods into a single (temporary)
> > helper stub.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  arch/x86/include/asm/nospec-branch.h |   19 +++++--------------
> >  arch/x86/lib/retpoline.S             |    7 +++++++
> >  2 files changed, 12 insertions(+), 14 deletions(-)
> > 
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -272,9 +272,9 @@
> >  .endm
> >  
> >  #ifdef CONFIG_CPU_UNRET_ENTRY
> > -#define CALL_ZEN_UNTRAIN_RET	"call zen_untrain_ret"
> > +#define CALL_UNTRAIN_RET	"call entry_untrain_ret"
> >  #else
> > -#define CALL_ZEN_UNTRAIN_RET	""
> > +#define CALL_UNTRAIN_RET	""
> >  #endif
> >  
> >  /*
> > @@ -293,15 +293,10 @@
> >  	defined(CONFIG_CALL_DEPTH_TRACKING) || defined(CONFIG_CPU_SRSO)
> >  	VALIDATE_UNRET_END
> >  	ALTERNATIVE_3 "",						\
> > -		      CALL_ZEN_UNTRAIN_RET, X86_FEATURE_UNRET,		\
> > +		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
> 
> SRSO doesn't have X86_FEATURE_UNRET set.

Argh.. this stuff doesn't exist at the end anymore, but yeah, that's
unfortunate.

I'll see if I can find another intermediate step.