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Date:   Sat, 20 Jan 2018 15:26:27 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Nadav Amit <nadav.amit@gmail.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>, w@1wt.eu,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC] x86: Avoid CR3 load on compatibility mode with PTI
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* Nadav Amit <nadav.amit@gmail.com> wrote:

> > So we are trading a 5-15% slowdown (PTI) for another 5-15% slowdown, plus we 
> > are losing the soft-SMEP feature on older CPUs that PTI enables, which is a 
> > pretty powerful mitigation technique.
> 
> This soft-SMEP can be kept by keeping PTI if SMEP is unsupported. Although we 
> trade slowdowns, they are different ones, which allows the user to make his best 
> decision.

Indeed, not allowing PTI to be disabled if SMEP is unavailable might be a 
solution.

> > Yes, I suspect in some (maybe many) cases it would be a speedup, but I really 
> > don't like the underlying assumptions and tradeoffs here. (Not that I like any 
> > of this whole Meltdown debacle TBH.)
> 
> To make sure that I understand correctly - the assumptions are that disabling 
> PTI on compatibility mode would: (1) Benefit some workloads; (2) Be useful, even 
> if we only consider CPUs with SMEP; and (3) Secure.
> 
> Under these assumptions, the tradeoff is slightly greater code complexity for 
> considerably better performance of 32-bit code; in some common cases this makes 
> 32-bit code to perform significantly better than 64-bit code.
> 
> Am I missing something? My main concern was initially security, but so far from 
> your aggregated feedback I did not see something concrete which cannot 
> relatively easily be addressed.

Yes, I suppose.

Thanks,

	Ingo