linux-kernel - Re: [PATCH 1/1] x86/speculation: Allow IBPB to be conditionally enabled on CPUs with always-on STIBP

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Message-ID: <20201104233143.GA2496945@google.com>
Date:   Wed, 4 Nov 2020 18:31:43 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "Anand K. Mistry" <amistry@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        tglx@...utronix.de, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Gross <mgross@...ux.intel.com>,
        Mike Rapoport <rppt@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Vineela Tummalapalli <vineela.tummalapalli@...el.com>,
        Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] x86/speculation: Allow IBPB to be conditionally
 enabled on CPUs with always-on STIBP

On Tue, Nov 03, 2020 at 11:57:57AM +0100, Borislav Petkov wrote:
> On Mon, Nov 02, 2020 at 11:02:10AM +1100, Anand K. Mistry wrote:
> > > I like the idea of passing in the mode you want to check, but it appears
> > > they are never used independently. The ibpb and stibp modes are always
> > > checked together in one of the if statements below, so you could make this
> > > a function that checks both modes and just have a single call. I'll leave
> > > that up to the maintainers to see what is preferred.
> > 
> > I can see both sides to this. Personally, I think I prefer it as-is
> > since I think it improves readability a bit by making the conditions
> > less complicated whilst not hiding too many details. I'll wait to see
> > what others say before changing this one.
> 
> Yes, but if you make it a single function with a descriptive name, you'd
> make the call sites even more readable:
> 
> 	if (!is_spec_ib_conditional(..))
> 		bla;
> 
> or
> 
> 	if (!is_spec_ib_user_controlled(..))
> 		blu;
> 
> and that function should simply check both spectre_v2_user_ibpb *and*
> spectre_v2_user_stibp in one go.
> 
> Why should we do that?
> 
> Exactly because you both got your brains twisted just from looking at
> this. Because this mitigation crap is such an ugly and complex maze that
> we would take even the smallest simplification any day of the week!
> 
> Welcome to my life since meltdown. Brain twist feels good, doesn't it?
> 
> :-)))

I hate the maze too. In theory we can get rid of STIBP if/when
core-scheduling is enabled because the cross-CPU branch predictor poisioning
would not be possible. Maybe that will simplify the maze a bit.

thanks,

 - Joel