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

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Date:   Wed, 9 Aug 2023 15:26:35 +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 03:12:43PM +0200, Peter Zijlstra wrote:
> 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.

I think simply setting UNRET for SRSO at this point will be sufficient.
That ensures the entry_untrain_ret thing gets called, and the
alternative there DTRT.

The feature isn't used anywhere else afaict.

Then later, after the fancy alternatives happen, this can be cleaned up
again.